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Epidemiology

EPIDEMIOLOGY

Epidemiology is the indispensable basic science of public health. It provides the logical framework for the facts that enable public health officials to identify important public health problems and to delineate their dimensions. Epidemiologic methods are used to define these health problems; to classify, identify, and elucidate their causes; and to plan and evaluate rational control measures.

HISTORICAL DEVELOPMENT OF EPIDEMIOLOGY

In ancient times, epidemics and plagues were terrifying natural phenomena that cried out for a more rational explanation than that they were due to the wrath of god or the machinations of evil spirits. Hippocrates (c. 460377 b.c.e.) described many kinds of epidemics and in On Airs, Waters, Places and other writings. He offered empirical insights into environmental and behavioral factors that might be associated with certain kinds of disease. Although doctors and others engaged in the healing arts did not clearly understand the concept of contagion until several hundred years later, Fracastorius (c. 14781553) identified several ways that infections can be transmittedby direct contact, by what we now call droplet spread, and by contaminated clothing.

The science of epidemiology took root with empirical observations of epidemics and other causes of death. John Graunt (16201674), in London, complied the first mortality tables on England's bills of mortality. Statistical analyses of deaths due to childbed fever by Ignaz Semmelweiss (18181865) in Vienna in the early nineteenth century and of tuberculosis by Pierre Charles Alexandre Louis (17871872) in Paris demonstrated the power of numbers. In London, in 1848 and 1854, meticulous, logical examination of the facts and figures about cholera epidemics by John Snow (18131858) revealed the mode of communication of this deadly epidemic disease. Snow is regarded as the founder of modern epidemiology because of his use of such careful methods.

Until early in the twentieth century almost all epidemiology focused on communicable diseases, although Percivall Pott's (17141788) observations on cancer of the scrotum in chimney sweeps and James Lind's dietary experiment with fresh fruit to prevent scurvy (1975) were precursors of modern noncommunicable disease epidemiology and clinical trials, respectively. The use of epidemiology in studies of coronary heart disease and cancer in large-scale trials of many new preventive and therapeutic regimens, in nationwide surveys of health status, and in evaluation of health services came to the fore in the second half of the twentieth century. In the final quarter of the twentieth century, powerful computers, information technology, and more rigorous methodological approaches transformed epidemiology and made it a mandatory feature of clinical science as well as the most fundamental basic science of public health.

DEFINITION AND SCOPE

The word "epidemiology" was coined in the mid nineteenth century to describe the scientific study of epidemics. Its meaning has expanded over the years, and present-day epidemiology encompasses the study of all varieties of illness and injury as they affect defined groups of people. In 1983 a committee representing the International Epidemiological Association defined epidemiology as "the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems." Study includes observation, surveillance, hypothesis-testing research projects, analysis of epidemiologic and other kinds of data, and certain other kinds of experiments. Distribution includes analysis of data according to the time scale over which events occur, the places where the events occur, and the categories of persons to whom they occur. Determinants are all the physical, biological, behavioral, social, and cultural factors that influence health. Health-related states or events include diseases, causes of death, behaviors such as the use of tobacco, reactions to preventive regimens, and provision and use of health services. Specified populations are those with identifiable characteristics such as known numbers and age groups. The ultimate aim and purpose of epidemiologyto promote, protect, and restore good healthis manifested in the "application of this study to control health problems."

Epidemiologists attempt to identify, measure, count, and control diseases, injuries, and causes of untimely death; and to relate these events to the associated inherited, environmental, and behavioral factors that cause or contribute to them. One of the great intellectual challenges of epidemiology is to dissect these factors and unravel their connections in order to identify exactly what is ultimately responsible for a particular disease or health problem.

RELATIONSHIP TO OTHER SCIENCES AND TECHNOLOGIES

The information used by epidemiologists comes from a diverse array of sources; draws on a wide range of sciences and technologies; and calls on the expertise of technologists and other people engaged in many kinds of crafts. Some connections are obviousthose with vital statistics, biostatistics, microbiology, immunology, and chemistry; with every clinical specialty from pediatrics to geriatrics and palliative care, and from family practice to hematology and neurosurgery. Other obvious connections are to the social and behavioral sciences, and, less obviously, to animal husbandry, wildlife biology, agricultural science, physics, atmospheric sciences, oceanography, engineering, town planning, education, law enforcement, communications technology, and the media. Epidemiology may be the most ecumenical of all the sciences. Probably no other branch of biomedical science has so many connections to such a wide range of other human activities.

RATES

The basis of all epidemiology is the comparison of groups of people. For these comparisons to be valid, it is necessary to convert raw numbers into rates. A rate is a fractionthe upper part (the numerator) is the number of people affected by the problem, event, or condition of interest; the lower part (the denominator) is the number of persons in the population who are at risk of experiencing the problem, event, or condition. Because the events normally continue over a long period, often indefinitely, rates are expressed in relation to a specified time. Since fractions are awkward to deal with, there is commonly a multiplier, and the rate, as shown in the following formula, is expressed in terms of so many per thousand, per hundred thousand, etc., in a specified time, usually a year, though shorter periods are used when circumstances warrant it:

In practice there are many variations in the ways rates are expressed, but the basic elements of events, population at risk, and time are common to all.

Rates have many uses. By comparing rates, epidemiologists can examine the experience of particular groups of people at specified times, in different cities, countries, or occupational groups. The observed differences are the basis for inferences about the reasons for these differences, and are used to test hypotheses about these reasons, possibly about the putative cause of a particular kind of cancer, for instance. In addition to the absolute requirement, for validity, of basing all comparisons on rates, another important use is in calculating the risks to individuals and groups of experiencing an event such as a heart attack, the occurrence of cancer, or traffic injury. Comparisons are often rendered invalid, or relatively unreliable, by differences among the populations being comparedoften because of failure to allow for various kinds of biases and confounding factors. A common problem stems from differences in the age composition of populations that are being compared. This problem is overcome by the procedure of age-adjustment. Another problem is that there may be important qualitative differences, such as health or employment status, between groups that are being compared.

The terms "incidence" and "prevalence" are often confused. Incidence refers to the number of new cases, events, or deaths, that occur in a specified time, usually one year. Prevalence refers to the total number of events or cases, both new and long-term, that are present at a particular point in time. Prevalence is therefore expressed as a number, not a rate, as there is no time dimension involved.

INVESTIGATING EPIDEMICS

An epidemic is the occurrence of a number of cases of a disease clearly in excess of normal expectation. This is usually a large number when the disease is one of the common infectious fevers, but even a single case of a dangerous contagious disease, such as typhoid, that has long been absent from a community should suffice to activate the highest level of epidemic surveillance and control measures. The occurrence of a small number of cases of a rare variety of cancer, closely clustered in time and space, may also signal an epidemic. Observational and analytic epidemiology blend in the investigation of epidemics. The investigation demands meticulous attention to detail in collecting information about all the cases of the condition, including mild and inconspicuous cases as well as those with florid manifestations, and must include details about all possible associated factors, such as dietary intake (this is especially important in outbreaks of food poisoning), occupation, living conditions, and unusual recent experiences. Particular attention is paid to the index casethe first identified case of a condition. In most infectious disease epidemics, this could be the case that introduced the infection into the affected community. Information is also gathered about healthy people in the same community, aimed at discovering why they have not been affected. Laboratory tests are used to confirm the diagnosis, identify the pathogenic organism, toxic chemical, or other agent that caused the disease; and to measure immunological responses among both sick and healthy people. Analyzing all this information often clarifies the nature and cause of an epidemic and points the way to appropriate control measures.

Investigating epidemics can be tedious because it needs to be so painstaking, even, seemingly, a boring routine task. But often it is as exciting as detective fiction. For example, an epidemic of typhoid in Aberdeen, Scotland, was traced eventually to a contaminated can of processed beef from Argentina. The can had been cooled in a river adjacent to the canning works. As the pressure inside the can fell when it cooled, a partial vacuum was created and typhoid bacilli in raw sewage in the water were sucked into the can through a minute hole.

Identifying the existence of an epidemic sometimes requires unusual vigilance and an ability to make connections among seemingly isolated events. An epidemic of lethal pneumonia among members of the American Legion who attended a convention in Philadelphia in 1976 and then returned to their hometowns before becoming ill, would not have come to light without rigorous scrutiny on the part of epidemic intelligence service officers of the Centers for Disease Control. Subsequent investigations led to the identification of Legionnaire's disease.

Techniques of molecular biology, notably DNA typing and the identification of biomarkers, have immensely enhanced the precision of epidemic investigation. It is now possible to trace the exact passage of an infectious agent such as the gonococcus or HIV (human immunodeficiency virus) as it is transmitted by direct contact from one individual to another among a group of people; or to show that coughing by a passenger with open pulmonary tuberculosis on a crowded airline flight can cause primary tuberculous infection of other passengers in the same compartment of that flight; or to determine how certain cancer-causing agents actually induce cancer. Books and articles in the popular press, notably the accounts by the journalist Berton Roueché in the New Yorker, and on some TV programs have communicated the excitement and challenge of epidemic investigations.

EPIDEMIOLOGIC METHODS

The application of several analytic methods of epidemiologic study has contributed substantially to scientists' understanding of disease causation, and therefore to control and prevention of many conditions of great public health importance. The available methods are observational epidemiology (the empirical study of naturally occurring events), analytic study, and, under carefully defined conditions and with all due ethical safeguards, human experimentation.

Observational Epidemiology. This method begins with surveillance of populations, using vital and health statisticsincluding analysis of death rates arranged by age, sex, locality, and cause of death. Other information is derived from notified cases of infectious diseases of public health importance, from registries of cancer or other diseases, and from hospital discharge statistics. Since 1957, the National Center for Health Statistics has conducted continuously a National Health Survey that has carried observational epidemiology to new levels of comprehensiveness.

It is often possible to make imaginative use of many other kinds of available information about defined population groups. Schools and many employers keep records of absences due to sickness, sometimes with reasons for these absences. Police and other law enforcement agencies keep records of calls to settle domestic disputes and of damage due to vandalism, which are useful indicators of social pathologies associated with local variations in the frequency of domestic violence, alcohol abuse, and broken families. All such sources of information combine to make it possible for epidemiologists and public health specialists to produce a multidimensional "community diagnosis." Serial measurements can indicate whether things are improving or getting worse, and in which ways these trends are moving for each of different indicators ranging from adolescent smoking behavior to reasons for long-term disability among the elderly.

Analytic Observational Studies. The possibilities of observational epidemiology are considerable, but not limitless. They are powerfully reinforced by analytic studies. The two main analytic methods are the case-control study and the cohort study.

Careful questioning of patients has enabled many doctors to make inferences about the influence of past experience on present disease. Percivall Pott, an eighteenth-century British physician, observed that cancer of the scrotum occurred among former chimney sweeps, and correctly inferred that it was associated with the accumulation of tar in the skin creases. Two hundred years later, in 1940, Norman Gregg, an ophthalmologist in Sydney, Australia, similarly inferred correctly that the cases he was seeing of congenital cataract must be associated with rubella (German measles), which their mothers had had during early pregnancy.

The case-control study is a systematic extension of routine medical history taking, in which the past histories of patients (the cases) suffering from the condition of interest are compared to the past histories of persons (the controls) who do not have the condition of interest, but who otherwise resemble the cases in such particulars as age and sex. Analysis of data about a series of cases and controls may show differences that are statistically significant. Sometimes only small numbers of cases are required to demonstrate significant differences between cases and controls. This makes the case-control study a suitable way to search for causes of rare conditions. For example, the discovery that a very rare form of liver cancer was strongly associated with occupational exposure to vinyl chloride required only four cases, and the fact that expectant mothers' use of artificial estrogens during early pregnancy can cause cancer of the vagina many years later in their daughters was based on a case-control study of eight cases. Although case-control studies can be flawed by the presence of biases that are often difficult or even impossible to eliminate, they are a valuable method of investigation because they can be done rapidly and at relatively little expense. The findings can be confirmed or refuted by more rigorous research methods such as cohort studies.

A cohort study is conducted by identifying individuals in a defined population who are exposed to varying levels of known or suspected risk for the condition of interest, such as cancer of the lung or coronary heart disease. The population is observed over a certain period, and the death and disease incidence rates among those exposed to varying and known levels of risk are compared. Cohort studies require large numbers, commonly many thousands, and prolonged observation, commonly years or even decades. They are therefore expensive, requiring a large and dedicated staff and maintenance of detailed records of very large numbers of people, only a small proportion of whom will ultimately fall ill and die of the condition of interest. Some cohort studies have become famous. The people of Framingham, Massachusetts, have been the subjects of cohort studies of coronary heart disease since 1948. In 1951, Richard Doll and Austin Bradford Hill began a cohort study of lung cancer in relation to tobacco smoking in a cohort of about 40,000 male British doctors. Later phases of this study have expanded to include risk factors for coronary heart disease and other chronic conditions; and by the late 1990s this study had yielded dramatic evidence of the relationship of tobacco smoking to cancers of many kindsand to coronary heart disease, chronic obstructive lung disease, and various other life-shortening chronic diseases.

It is possible to get results from a cohort study without waiting many years, if detailed information about exposure to risk factors at some time in the past is available in sufficient detail for a population of sufficient size. A method that permits reliable linking of past and present medical and other relevant records, such as a record linkage system, facilitates this approach. Record linkage is the process of relating information from two or more sets of recordscompiled years apart and sometimes by different agenciesabout the same individuals. A prerequisite is a way to identify individuals with a high degree of precision, such as a unique numbering system, or a system combining a sequence of digits for birthdate, birthplace, and sex; with alphabet letters or a phonetic code used for other details, such as the individual's mother's maiden name. Obviously the logistics of all this make it a costly method, but the yield can justify the expense. This method, known as an historical cohort study, has demonstrated the relationship of childhood cancer and developmental anomalies to prenatal maternal exposure to small diagnostic doses of X-rays. Record linkage and historical cohort studies have also demonstrated a relationship between birthweight and the occurrence of cardiovascular disease in middle age.

Experimental Epidemiology. In the 1920s, experimental epidemiology meant observing the passage of infectious pathogens in colonies of rodents, but such experiments are rarely necessary, and the meaning of the term has changed. Experiments in which the investigator studies the effects of intentional alteration or intervention in the course of a disease are now done on humans rather than experimental animals, usually using a randomized controlled-trial design.

The randomized controlled trial (RCT) is a form of human experimentation in which the subjects, usually patients, are randomly allocated to receive either a standard accepted therapeutic or preventive regimen, or an experimental regimen. The purpose of random allocation is to eliminate or minimize bias in the selection of subjects. This greatly enhances the validity of the results. Preferably, the subjects and those observing the trial's results should be unaware of which subjects are receiving the experimental and control regimens, thus eliminating the power of suggestion as a factor influencing the response of individuals to the regimen. There are very important ethical constraints on the conduct of randomized controlled trials. The only ethically acceptable justification for conducting a randomized controlled trial is uncertainty about which of the available regimens is the best, a state of affairs known as "equipoise." It is absolutely essential to obtain the genuinely informed consent of all human subjects on whom a trial is conducted.

CLINICAL EPIDEMIOLOGY AND EVIDENCE-BASED MEDICINE

In the final quarter of the twentieth century, physicians in clinical practice discovered the value of epidemiologic methods in enhancing the efficacy of treatment regimens, mainly through rigorous attention to the nature and quality of the evidence on which clinical decisions are based. Evidencebased medicine then moved into public health practice, where it is illuminating decisions about many aspects of public health practice, such as the most effective way to deploy public health nurses in a local health department.

OTHER RECENT ADVANCES

Epidemiology made spectacular progress in several other directions in the 1990s. One was in the application of molecular biology, resulting in what is sometimes called molecular epidemiology. Other advances have been made in genetic epidemiology, where the meeting of molecular genetics with public health, occupational and environmental health, and infant and child health has produced both exciting stories of great progress and difficult ethical and moral problems. What are scientists and physicians to do, for instance, with the newfound knowledge and technical capability to identify defective genes, especially genes that, in interaction with some environmental circumstances, can disqualify certain individuals from particular occupations and can render others ineligible for life insurance? Such dilemmas presage a testing time for society's values.

Another set of new challenges face epidemiologists who specialize in studies of risk management. The global environment is changing as the burden of greenhouse gases increases and leads to a rise in average global ambient temperatures, and remote sensing and climate models enable us to predict the likely future distribution of vector-borne diseases such as malaria, dengue, and schistosomiasis. A new realm of risk factor analysis is thus emerging, based on future health scenarios that incorporate climate models and in the most sophisticated applicationsinclude sets of models for future patterns of biodiversity, human settlements, and economic and industrial dynamics. In these ways epidemiologists are helping to plan the public health services that will be needed in the future.

John M. Last

(see also: Case-Control Study, Cohort Study, Cross-Sectional Study; Epidemiologic Transition; Graunt, John; Hippocrates of Cos; Mortality Rates; Notifiable Diseases; Pott, Percivall; Rates; Rates: Age-Adjusted; Record Linkage; Semmelweiss, Ignaz; Snow, John; Vital Statistics; and other articles on specific diseases mentioned herein )

Bibliography

Ashton, J., ed. (1994). The Epidemiological Imagination. Buckingham, UK: Open University Press.

Beaglehole, R.; Bonita, R.; and Kjellström, T. (1993). Basic Epidemiology. Geneva: World Health Organization.

Buck, C.; Llopis, A.; Nájera, E.; and Terris, M., eds. (1988). The Challenge of Epidemiology. Washington, DC: Pan American Health Organization.

Last, J. M., ed. (2000). A Dictionary of Epidemiology, 4th edition. New York: Oxford University Press.

Rothman, K. J., and Greenland, S., eds. (1998). Modern Epidemiology, 2nd edition. Philadelphia: Lippincott-Raven.

Roueché, B. (1954). Eleven Blue Men, and Other Narratives of Medical Detection. Boston: Little, Brown & Co.

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Epidemiology

Epidemiology

BIBLIOGRAPHY

Epidemiology is a branch of ecology that includes both the sum of what is known concerning the differential distribution of disease throughout a population and the techniques for collecting and analyzing data dealing with the prevalence and incidence of disease among different social groups. While originally limited to the study of epidemics or the spread of contagious disease, epidemiology today covers all types of disease, degenerative as well as communicable, and all population characteristics—social and psychological as well as biological and physical—that may help to describe or explain the prevalence of disease.

Methods of epidemiology . In the broad sense of the term, epidemiology deals with the occurrence and distribution of disease among different population groups, whether human, animal, or plant. The discovery or description of these differences has been called descriptive, or comparative, epidemiology, whereas the analysis of the causal factors and conditions producing these differences is usually referred to as explanatory, or analytic, epidemiology. As epidemiology becomes increasingly concerned with the study of the origin and course of disease, rather than solely with its distribution, this distinction is gradually disappearing.

Because of its emphasis upon the relationship between environmental factors and disease, epidemiology is properly regarded as a major branch of human ecology, or “the study of the relations between man and his environment, both as it affects him and as he affects it” (Rogers 1960, p. vii). In general, three main sets of interacting factors form the focus of epidemiological interest: the host, or human individual varying in genetic resistance, susceptibility, and degree of immunity to the disease; the agent, or carrier of the disease, including any adverse process, whether it be an excess, deficiency, or interference of a microbial, toxic, or metabolic factor, and varying according to infectivity, virulence, and pathogenesis; and the environment, or surrounding medium, social as well as physical, which affects both the susceptibility of the host, the virulence of the agent or disease process, and the quantity and quality of contact between host and agent (Paul 1950, pp. 53-54). These three sets of factors do not exist in any simple one-to-one relationship but maintain a complex, ever-changing balance. The occurrence of disease, especially mass disease, is the result of a multiplicity of causal factors, each of which contributes to, rather than accounts for, the appearance of the disease.

Epidemiological knowledge consists of the available facts and theories concerning the relationships between these three factors and the various disease entities and health conditions. Social epidemiology, as a subdivision of epidemiology, concentrates on the social, as opposed to the physical or biological, factors in the incidence and prevalence of disease. In the case of the chronic, degenerative diseases and the mental and behavioral disorders, both of which constitute primary targets of modern epidemiology, distinctions between host, agent, and environmental factors and between social and biological or physical factors are becoming increasingly difficult to maintain.

As a research method, epidemiology refers to “the application of scientific principles to investigations of conditions affecting groups in the population [constructive epidemiology]” (Clark [1953] 1958, p. 65). Predominantly, this involves the observation of the occurrence of disease under natural conditions in whole populations, as opposed to clinical or laboratory investigations. Epidemiological method, for the most part, uses the research techniques of the population survey to discover the relationship between the occurrence of disease and the presence of various biological, physical, and social factors. The kind of “proof” that it tries, for the most part, to obtain is statistical association between the presumed “causal” factor and the occurrence of the disease. Dawber and Kannel (1963, pp. 433-434) have spoken of “macroscopic” studies, which correlate rates of a disease with other statistical measures for an area or population group (ecological correlations), as contrasted with “microscopic” studies, which correlate personal characteristics with the presence or absence of disease within the individual (individual correlations). Experimental epidemiology, involving the controlled introduction of epidemic conditions into populations of experimental animals in the laboratory (Greenwood 1932), field experiments to test the efficacy of various immunizing agents, or various types of preventive measures (MacMahon et al. 1960, pp. 268-279), represents an attempt to apply the experimental method to epidemiological problems.

Historical background . The scope of epidemiology, which was “originally concerned only with epidemics, … was extended first to include infectious diseases which do not ordinarily occur in epidemic form, such as leprosy, syphilis, and tuberculosis, and later to noninfectious diseases” (Doull 1952, p. 76). The birth of epidemiology as we know it may be traced back to England in the late seventeenth century, when John Graunt in 1662 developed the first mortality tables. However, it was not until the mid-nineteenth century that men like Johann Sussmilch and Adolphe Quetelet utilized these statistics to help identify etiological factors in disease. The major emphasis of epidemiology under such eminent pioneers as John Snow (cholera), Peter Panum (measles), William Budd (typhoid), and Kenneth Maxcy (endemic typhus) was upon the discovery of host, agent, and environmental factors associated with the spread of these highly contagious diseases, or what has been called “the mass-phenomena of infectious diseases” (see Frost 1910-1939).

The dramatic conquest of the infectious diseases in the present century, together with the growing importance of the chronic, degenerative diseases, soon made it apparent that epidemiology could no longer be restricted to epidemics. As a matter of fact, epidemiological studies of nutritional (James Lind on scurvy) and occupational (Henry B. Baker on lead colic) diseases had already demonstrated the applicability of epidemiological method to noninfectious diseases. The use of statistical associations based upon population surveys became one of the foremost methods for studying the occurrence of cancer, cardiovascular disease, and mental illness and for the difficult task of identifying specific etiological agents. Today, the value of epidemiological research for the study of all diseases is well established (James & Greenberg 1957).

Uses of epidemiology . As a standard tool of medical investigation, epidemiology has been brought to bear upon almost all aspects of the prevention and treatment of disease. Morris (1957) has listed seven fundamental applications: the determination of individual risks on the basis of morbidity tables and cohort analysis—for example, the chances of a forty-year-old male getting cancer; the securing of data on subclinical and undetected cases; the identification of syndromes or clusters of symptoms; the determination of historical trends of disease; the diagnosis of community health needs and resources; program planning, operation, and evaluation; and the search for causes of disease. Similar uses are described by Breslow (1957) for a large-scale epidemiological survey of chronic diseases in California. These include a demographic description of the changing population composition, a broad picture of the state of health and illness in the community, more extensive knowledge about disease prevalence, data on the utilization of health services, case rosters for follow-up investigations, and data on etiological factors. Thus, epidemiology provides a large portion of the scientific base for public health practice.

The diversity of these applications would suggest that epidemiological surveys are often combined, or confused, with general community health surveys. A survey that asks questions about health conditions and medical care of a population sample does not automatically become an epidemiological study. From a more rigorous point of view, the major contribution of epidemiological research should be in the development and testing of hypotheses concerning specific factors that may influence the distribution of some particular disease in a defined population. On the basis of existing knowledge, theory, or observation, the epidemiologist identifies subgroups of the population believed to have varying incidence rates of the disease being investigated. He then hypothesizes certain etiological factors related to the disease and also believed to differ among the subgroups being studied. By means of a field survey or the analysis of existing data, he then tests the direction and degree of association between the occurrence of the disease and the presence or absence of the group characteristic hypothesized as the etiological factor.

Epidemiology and social science . Epidemiology has theoretical and methodological ties to the social sciences. Both the epidemiologist and the social scientist are concerned with demography and ecology—the relationship of man to his environment (Fleck & lanni 1958). When the environment includes sociocultural factors as possible “causes” of disease, either indirectly (as in the case of poverty leading to malnutrition or unsanitary living conditions) or directly (as in the case of emotional disturbance leading to mental disease or addictive disorders, such as drinking and alcoholism or drug addiction), then all three basic components of epidemiology—host, agent, and environment—take on important social dimensions (King 1963). Epidemiology is becoming increasingly concerned with “the social component of environment … that part which results from the association of man with his fellow man … the attainments, beliefs, customs, traditions, and like features of a people” (Gordon 1952, pp. 124-125). In the current era of chronic, degenerative diseases, in which an individual’s whole way of life may become more important than any single infectious agent in the disease process, social factors become a primary target for epidemiological investigation.

Methodologically, both the epidemiologist and the social scientist rely heavily upon the population survey and field experiment. Similar problems of research design confront both groups, while technical considerations such as sampling, questionnaire construction, interviewing, and multivariate analysis are objects of mutual methodological interest (Wardwell & Bahnson 1964).

Recent research . All major diseases today are the subject of epidemiological research, and almost all of these include, at the minimum, such social groupings as sex, age, marital status and family composition, occupation, socioeconomic status, religion, and race. In addition, many studies are specifically aimed at the investigation of social factors, such as social stress, as possible etiological agents in the occurrence of the disease. Comprehensive reviews have been prepared by Clock and Lennard (1956) on hypertension, Graham (1960) on cancer, Mishler and Scotch (1963) on schizophrenia, Dawber and others (1959) on heart disease, Jaco (1960) and Hoch and Zubin (1961) on mental disease, Suchman and Scherzer (1960) on childhood accidents, King and Cobb (1958) on rheumatoid arthritis, among others. The state of knowledge in this field is advancing rapidly, and the findings of epidemiological surveys appear regularly in such periodicals as the American Journal of Public Health and the Journal of Chronic Diseases.

In general, these studies reveal a large number of significant differences in the occurrence of disease among different subgroups of the population (Pemberton 1963). For example, coronary artery disease is found to vary according to such sociocultural variables as occupation, economic status, race, and rural-urban residence. Cancer of the uterine cervix occurs much less frequently among Jewish women; men are more likely to incur cardiovascular disease; and mental illness is found more often among the lower socioeconomic groups. On a more psychological level, insecurity and stress tend to be associated with a higher incidence of mental illness, alcoholism, narcotics addiction, heart disease, arthritis, and a host of psychosomatic conditions (Leighton 1959). Perhaps the most famous of these epidemiological correlations deals with the association between smoking behavior and lung cancer (Dorn & Cutler 1958).

Some problems of research design . The major conceptual and methodological problems in epidemiological research stem from its dependence, by and large, upon associational evidence. The basic research design of epidemiological method consists in the comparison of two groups, each with varying rates of a disease, with respect to other characteristics hypothesized as explanatory of these varying disease rates. This is essentially an ex post facto form of survey research and one that may undertake demographic studies of existing vital statistics or several other types of study using data specially gathered for the purpose. These can be classified as being either retrospective studies, which secure data on different group characteristics hypothesized as etiological factors from at least two groups with varying rates of the disease being investigated, or prospective studies, which follow up groups of individuals with and without the hypothesized etiological characteristics in order to determine the differential development of the disease.

In all three study designs, the objective is the determination of a series of statistical associations from which etiological inferences may be drawn. These three types of design offer progressively more rigorous and plausible evidence of causality. The demographic method, relying as it does on ecological correlations, is the weakest, since variations in rates of occurrence between phenomena do not necessarily mean that these phenomena are related (Clausen & Kohn 1954); it is possible to have high ecological associations with little or no individual correlation. Retrospective studies do provide individual correlations, but there is often no way of knowing which of the two factors in an observed correlation came first. Prospective studies using a longitudinal study of cohorts are strongest, since these enable one to define the population at risk in advance of the development of disease and then to check one’s predictions over time [see COHORT ANALYSIS].

Smoking and lung cancer. The association between smoking and lung cancer provides an excellent example of the progression from demographic to retrospective and finally to prospective studies. The initial association was suggested by demographic comparisons showing a much higher incidence of lung cancer among men than women. Retrospective studies revealed a correlation between smoking histories and the occurrence of lung cancer. Finally, intensive prospective studies following up smokers and nonsmokers showed a higher development of lung cancer among the former. The continuing controversy today, however, demonstrates the further need and demand to prove, through experimental rather than epidemiological studies, that smoking can “cause” cancer.

Validity of epidemiological method. The inability of the epidemiologist to “randomize” his experimental and control groups and to alter deliberately the characteristics of his experimental group constitutes an intrinsic conceptual and methodological shortcoming that requires a continuing close working relationship between epidemiological and experimental research. Certain basic prerequisites must be satisfied if epidemiological method is to produce reliable and valid associations. First, the representativeness and generalizability of the sample from whom data are obtained must be ascertainable. This sample should include not only persons who are known to have the disease but also who are free of the disease. The definition of what is a “normal,” or disease-free, control group presents a particularly difficult problem for epidemiological study of the chronic diseases, since these may not become apparent until a fairly late stage. Second, the disease being studied must be defined in such a way that it can be reliably and validly diagnosed using field techniques. Errors due to false positives (the proportion of individuals classified as diseased among those truly not diseased) and false negatives (the proportion classified as not diseased among those truly diseased) can often lead to spurious associations (Rubin et al. 1956). Third, the hypothesized etiological factors must be similarly capable of objective definition and measurement. These are difficult conditions to meet, especially in relation to the chronic diseases, which often lack both clear-cut diagnostic criteria and well-developed theories of etiology and process (Pollack & Krueger 1960).

Future developments . Epidemiological method is bound to increase in importance as the search for etiological factors in the chronic diseases forces the medical researcher to supplement his laboratory experiments with field studies, both as source and proof of his hypotheses. The multiple nature of etiological factors (many, if not most, of which cannot be reproduced or controlled in the laboratory) will require greater reliance upon population surveys and field trials. Probabilities of disease will replace certainties, and associated conditions rather than specific causes will dominate the picture. Prominent among these conditions will be the cultural, social, and psychological forces that determine how man lives and which in later years influence the degenerative processes. Today we deal with these social factors on the most elementary level, that of descriptive group memberships. Tomorrow we may hope to be able to determine the dynamic factors underlying these group memberships and to develop and test specific hypotheses of how and why social factors relate to the origin and course of disease.

Edward A. Suchman

[See alsoDRINKING AND ALCOHOLISM; DRUGS, article onDRUG ADDICTION: SOCIAL ASPECTS; ECOLOGY, article onHUMAN ECOLOGY; PUBLIC HEALTH; VITAL STATISTICS; and the biographies ofGRAUNTandQUETELET.]

BIBLIOGRAPHY

Breslow, Lester 1957 Uses and Limitations of the California Health Survey for Studying the Epidemiology of Chronic Disease. American Journal of Public Health 47:168-172.

CLARK, E. GURNEY (1953) 1958 An Epidemiological Approach to Preventive Medicine. Chapter 3 in Hugh R. Leavell et al., Preventive Medicine for the Doctor in His Community: An Epidemiologic Approach. 2d ed. New York: McGraw-Hill.

Clausen, John A.; and KOHN, MELVIN L. 1954 The Ecological Approach in Social Psychiatry. American Journal of Sociology 60:140-151.

Dawber, Thomas R.; and KANNEL, WILLIAM B. 1963 Coronary Heart Disease as an Epidemiology Entity. American Journal of Public Health 53:433-437.

Dawber, Thomas R. et al. 1959 Some Factors Associated With the Development of Coronary Heart Disease. American Journal of Public Health 49:1349-1356.

Dorn, Harold F.; and CUTLER, SIDNEY J. 1958 Morbidity From Cancer in the United States. U.S. Public Health Service Publication No. 590; Public Health Monograph No. 56. Washington: Public Health Service.

Doull, James A. 1952 The Bacteriological Era (1876-1920). Pages 74-113 in Franklin H. Top (editor), The History of American Epidemiology. St. Louis, Mo.: Mosby.

Fleck, Andrew C.; and IANNI, FRANCIS A. J. 1958 Epidemiology and Anthropology: Some Suggested Affinities in Theory and Method. Human Organization 16, no. 4:38-40.

Frost, Wade Hampton (1910-1939) 1941 Papers of Wade Hampton Frost, M.D.: A Contribution to Epidemiological Method. Edited by Kenneth F. Maxcy. New York: Commonwealth Fund; Oxford Univ. Press. → These essays provide a brilliant description of the transition to modern epidemiology.

Glock, Charles Y.; and LENNARD, HENRY L. 1956 Studies in Hypertension. Journal of Chronic Diseases 5:178-196.

Gordon, John E. 1952 The Twentieth Century—Yesterday, Today, and Tomorrow (1920). Pages 114-167 in Franklin H. Top (editor), The History of American Epidemiology. St. Louis, Mo.: Mosby. → Contains a comprehensive bibliography and discussion of modern developments.

Graham, Saxon 1960 Social Factors in the Epidemiology of Cancer at Various Sites. New York Academy of Sciences, Annals 84:807-815.

Greenwood, Major 1932 Epidemiology, Historical and Experimental. Baltimore: Johns Hopkins Press; Oxford Univ. Press.

Hoch, Paul H.; and ZUBIN, JOSEPH (editors) 1961 Comparative Epidemiology of the Mental Disorders. Proceedings of the 49th annual meeting of the American Psychopathological Association, February 1959. New York: Grune & Stratton.

JACO, E. GARTLY 1960 The Social Epidemiology of Mental Disorders: A Psychiatric Survey of Texas. New York: Russell Sage Foundation.

JAMES, GEORGE; and GREENBERG, MORRIS 1957 The Medical Officer’s Bookshelf on Epidemiology and Evaluation. Part 1: Epidemiology. American Journal of Public Health 47:401-408. → Contains a brief review and bibliography on the epidemiology of various diseases.

King, Stanley H. 1963 Social Psychological Factors in Illness. Pages 99-121 in Howard E. Freeman et al. (editors), Handbook of Medical Sociology. Englewood Cliffs, N.J.: Prentice-Hall.

King, Stanley H.; and COBB, SIDNEY 1958 Psychosocial Factors in the Epidemiology of Rheumatoid Arthritis. Journal of Chronic Diseases 7:466-475.

Leighton, Alexander H. 1959 My Name Is Legion: Foundations for a Theory of Man in Relation to Culture. The Stirling County Study of Psychiatric Disorder and Sociocultural Environment, Vol. 1. New York: Basic Books. → Contains a theoretical discussion of social stress as a factor in mental illness.

MACMAHON, BRIAN; PUGH, THOMAS F.; and IPSEN, JOHANNES 1960 Epidemiologic Methods. Boston: Little. → Contains a critical review of current concepts and methods.

Mishler, Elliot G.; and SCOTCH, NORMAN A. 1963 Sociocultural Factors in the Epidemiology of Schizophrenia. Psychiatry 26:315-351.

Morris, Jeremy N. 1957 Uses of Epidemiology. Baltimore: Williams & Wilkins; Edinburgh: Livingstone.

Paul, John R. 1950 Epidemiology. Pages 52-62 in David E. Green and W. Eugene Knox (editors), Research in Medical Science. New York: Macmillan.

Pemberton, John (editor) 1963 Epidemiology: Reports on Research and Teaching, 1962. Oxford Univ. Press.

POLLACK, HERBERT; and KRUEGER, DEAN E. (editors) 1960 Epidemiology of Cardiovascular Diseases: Methodology. American Journal of Public Health 50 (Supplement): 1-124.

Rogers, Edward S. 1960 Human Ecology and Health: An Introduction for Administrators. New York: Macmillan.

RUBIN, THEODORE; ROSENBAUM, JOSEPH; and COBB, SIDNEY 1956 The Use of Interview Data for the Detection of Associations in Field Studies. Journal of Chronic Diseases 4:253-266.

Suchman, Edward A.; and SCHERZER, ALFRED L. 1960 Current Research in Childhood Accidents. Part 1 in Association for the Aid of Crippled Children, Two Reviews of Accident Research. New York: The Association.

U.S. SURGEON GENERAL’S ADVISORY COMMITTEE ON SMOKING AND HEALTH 1964 Smoking and Health. U.S. Department of Health, Education and Welfare, Public Health Service Publication No. 1103. Washington: Government Printing Office. → Contains a thorough analysis of the epidemiological evidence on smoking as a cause of cancer and other diseases.

Wardwell, Walter I.; and BAHNSON, CLAUS B. 1964 Problems Encountered in Behavioral Science Research in Epidemiological Studies. American Journal of Public Health 54:972-981.

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Epidemiology

Epidemiology

Epidemiology is the study of the occurrence, frequency, and distribution of diseases in a given population. As part of this study, epidemiologistsscientists who investigate epidemics (widespread occurrence of a disease that occurs during a certain time)attempt to determine how the disease is transmitted, and what are the host(s) and environmental factor(s) that start, maintain, and/or spread the epidemic.

Epidemiology can be an important facet of a forensic investigation. A recent infamous example occurred in the fall of 2001, when a number of letters containing spores of Bacillus anthracis, the agent that causes anthrax , were sent through the United States postal system. The illnesses and deaths that resulted prompted the near shut-down of the postal delivery system, and an investigation to find the sender(s) of the letters and the source of the bacterial spores. These investigations were rooted in epidemiology.

The primary focus of epidemiology is on groups of persons, rather than individuals. The primary effort of epidemiologists is in determining the etiology (cause) of the disease and identifying measures to stop or slow its spread. This information, in turn, can be used to create strategies by which the efforts of health care workers and facilities in communities can be most efficiently allocated for this purpose.

In tracking a disease outbreak, epidemiologists may use any or all of three types of investigation: descriptive epidemiology, analytical epidemiology, and experimental epidemiology.

Descriptive epidemiology is the collection of all data describing the occurrence of the disease, and usually includes information about individuals infected, and the place and period during which it occurred. Such a study is usually retrospective, i.e., it is a study of an outbreak after it has occurred. The 2001 anthrax investigation is one example.

Analytical epidemiology attempts to determine the cause of an outbreak. Using the case control method, the epidemiologist can look for factors that might have preceded the disease. Often, this entails comparing a group of people who have the disease with a group that is similar in age, sex, socioeconomic status, and other variables, but does not have the disease. In this way, other possible factors, e.g., genetic or environmental, might be identified as factors related to the outbreak.

Using the cohort method of analytical epidemiology, the investigator studies two populations, one who has had contact with the disease-causing agent and another that has not. For example, the comparison of a group that received blood transfusions with a group that has not might disclose an association between blood transfusions and the incidence of a blood borne disease, such as hepatitis B.

Experimental epidemiology tests a hypothesis about a disease or disease treatment in a group of people. This strategy might be used to test whether or not a particular antibiotic is effective against a particular disease-causing organism. One group of infected individuals is divided randomly so that some receive the antibiotic and others receive a placeboa "false" drug that is not known to have any medical effect. In this case, the antibiotic is the variable, i.e., the experimental factor being tested to see if it makes a difference between the two otherwise similar groups. If people in the group receiving the antibiotic recover more rapidly than those in the other group, it may logically be concluded that the variableantibiotic treatmentmade the difference. Thus, the antibiotic is effective.

In the process of studying the cause of an infectious disease, epidemiologists often view it in terms of the agent of infection (e.g., particular bacterium or virus), the environment in which the disease occurs (e.g., crowded slums), and the host (e.g., hospital patient). Another way epidemiologists may view etiology of disease is as a "web of causation." This web represents all known predisposing factors and their relations with each other and with the disease. For example, a web of causation for myocardial infarction (heart attack) can include diet, hereditary factors, cigarette smoking, lack of exercise, susceptibility to myocardial infarction, and hypertension. Each factor influences and is influenced by a variety of other factors.

Epidemiologic investigations are largely mathematical descriptions of persons in groups, rather than individuals. The basic quantitative measurement in epidemiology is a count of the number of persons in the group being studied who have a particular disease; for example, epidemiologists may find 10 members of a village in the African village of Zaire suffer from infection with Ebola virus infection; or that 80 unrelated people living in an inner city area have tuberculosis.

A fundamental underpinning of infectious epidemiology is the confirmation that a disease outbreak has occurred. Once this is done, the disease is followed with time. The pattern of appearance of cases of the disease can be tracked by developing what is known as an epidemic curve. This information is vital in distinguishing a natural outbreak from a deliberate and hostile act, for example. The appearance of a few cases at first with the number of cases increasing over time to a peak is indicative of a natural outbreak. The number of cases usually begins to subside as the population develops immunity to the infection (e.g., influenza). However, if a large number of cases occur in the same area at the same time, the source of the infection might not be natural. Examples include a food poisoning or a bioterrorist action where the accidental or deliberate release of organisms will be evident as a sudden appearance of a large number of cases at the same time.

Any description of a group suffering from a particular disease must be put into the context of the larger population. This shows what proportion of the population has the disease. The significance of ten people out of a population of 1,000 suffering tuberculosis is vastly different, for example, than if those ten people were part of a population of one million.

Thus one of the most important tasks of the epidemiologist is to determine the prevalence ratethe number of persons out of a particular population who have the disease (prevalence rate). A prevalence rate can represent any time period, e.g., day or hour; and it can refer to an event that happens to different persons at different times, such as complications that occur after drug treatment (on day five for some people or on day two for others).

The incidence rate is the rate at which a disease develops in a group over a period of time. Rather than being a snapshot, the incidence rate describes a continuing process that occurs over a particular period of time.

Period prevalence measures the extent to which one or all diseases affects a group during the course of time, such as a year.

Epidemiologists also measure attributable risk, which is the difference between two incidence rates of groups being compared, when those groups differ in some attribute that appears to cause that difference. For example, the lung cancer mortality rate among a particular population of non-smoking women 50 to 70 years old might be 20/100,000, while the mortality rate among woman in that age range who smoke might be 150/100,000. The difference between the two rates (150 20 = 130) is the risk that is attributable to smoking, if smoking is the only important difference between the groups regarding the development of lung cancer.

Epidemiologists arrange their data in various ways, depending on what aspect of the information they want to emphasize. For example, a simple graph of the annual occurrence of viral meningitis might show by the "hills" and "valleys" of the line in which years the number of cases increased or decreased. This might provide evidence of the cause and offer ways to predict when the incidence might rise again.

Bar graphs showing differences in rates among months of the year for viral meningitis might pinpoint a specific time of the year when the rate goes up, for example, in summertime. That, in turn, might suggest that specific summertime activities, such as swimming, might be involved in the spread of the disease.

One of the most powerful tools an epidemiologist can use is case reporting: reporting specific diseases to local, state, and national health authorities who accumulate the data. Such information can provide valuable leads as to where, when, and how a disease outbreak is spread, and help health authorities to determine how to halt the progression of an epidemicone of the most important goals of epidemiology.

Molecular epidemiology has been used to trace the cause of bacterial, viral, and parasitic diseases. This knowledge is valuable in developing a strategy to prevent further outbreaks of the microbial illness, since the probable source of a disease can be identified.

Molecular epidemiology arises from varied scientific disciplines, including genetics, epidemiology, and statistics. The strategies involved in genetic epidemiology encompass population studies and family studies. Sophisticated mathematical tools are now involved, and computer technology is playing a predominant role in the development of the discipline. Multidisciplinary collaboration is crucial to understanding the role of genetic and environmental factors in disease processes.

Much information can come from molecular epidemiology, even in the exact genetic cause of the malady is not known. For example, the identification of a malady in generations of related people can trace the genetic characteristic, and even help identify the original source of the trait. This approach is commonly referred to as genetic screening. The knowledge of why a particular malady appears in certain people, or why such people are more prone to a microbial infection than other members of the population, can reveal much about the nature of the disease in the absence of the actual gene whose defect causes the disease.

Various routes can spread infections (i.e., contact, air borne, insect borne, food and water intake, etc.). Likewise, the route of entry of an infectious microbe can also vary from microbe to microbe.

Laboratory analysis techniques can be combined with other techniques to provide information related to the spread of an outbreak. For example, microbiological data can be combined with geographic information systems (GIS ). GIS information has helped pinpoint the source of outbreaks. In addition to geographic based information, epidemiologists will use information including the weather on the days preceding an outbreak, mass transit travel schedules, and schedules of mass-participation events that occurred around the time of an outbreak to try an establish a pattern of movement or behavior to those who have been affected by the outbreak. Use of credit cards and bank debit cards can also help piece together the movements of those who subsequently became infected.

Reconstructing the movements of people is especially important when the outbreak is of an infectious disease. The occurrence of the disease over time can yield information as to the source of an outbreak.

Epidemiologists were among the first scientists to effectively utilize the Internet and email capabilities to effectively communicate regarding disease outbreaks. The International Society for Infectious Diseases sponsors PROMED, a global e-mail based electronic reporting system for outbreaks of emerging infectious diseases and toxins , which is open to all sources.

see also Anthrax, investigation of 2001 murders; Ebola virus; Pathogens; September 11, 2001, terrorist attacks (forensic investigations of).

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Epidemiology

Epidemiology

ANTONIO FARINA/

BRIAN D. HOYLE

Epidemiology is the study of the various factors that influence the occurrence, distribution, prevention, and control of disease, injury, and other health-related events in a defined human population. By the application of various analytical techniques including mathematical analysis of the data, the probable cause of an infectious out-break can be pinpointed. This connection between epidemiology and infection makes microorganisms an important facet of epidemiology, and gives epidemiologists a vital link in emergency planning for public health response to a biological attack.

Molecular epidemiology has been used to trace the cause of bacterial, viral, and parasitic diseases. This knowledge is valuable in developing a strategy to prevent further outbreaks of the microbial illness, since the probable source of a disease can be identified.

Furthermore, in the era of biological weapons use by individuals, organizations, and governments, epidemiological studies of the effect of exposure to infectious microbes has become more urgently important. Knowledge of the effect of a bioweapon on the battlefield may not extend to the civilian population that might also be secondarily affected by the weapons. Thus, epidemiology is an important tool in identifying and tracing the course of an infection.

Molecular and genetic basis of epidemiology. Genetic epidemiology studies could result in data that would enable forensic investigators to rapidly identify bioterrorism or biological warfare agents specifically engineered or vectored to affect certain subgroups within a larger population.

Molecular epidemiology arises from varied scientific disciplines, including genetics, epidemiology and statistics. The strategies involved in genetic epidemiology encompass population studies and family studies. Sophisticated mathematical tools are now involved, and computer technology is playing a predominant role in the development of the discipline. Multidisciplinary collaboration is crucial to understanding the role of genetic and environmental factors in disease processes.

Much information can come from molecular epidemiology even if the exact genetic cause of the malady is not known. For example, the identification of a malady in generations of related people can trace the genetic characteristic, and even help identify the original source of the trait. This approach is commonly referred to as genetic screening. The knowledge of why a particular malady appears in certain people, or why such people are more prone to a microbial infection than other members of the population, can reveal much about the nature of the disease in the absence of the actual gene whose defect causes the disease.

Differences in response to pathogens is often a complex interplay of various environmental and genetic factors that require sophisticated analytical tools and techniques to identify. Aided by advances in computer technology, scientists develop complex mathematical formulas for the analysis of epidemiological models, the description of the transmission of the disease, and genetic-environmental interactions. Sophisticated mathematical techniques are now used for assessing classification, diagnosis, prognosis and treatment of many diseases.

Population studies provide data that greatly impact public health programs and emergency responses. By means of several statistical tools, genetic epidemiologic studies evaluate risk factors, inheritance and possible models of inheritance. Different kinds of studies are based upon the number of people who participate and the method of sample collection (i.e., at the time of an outbreak or after an outbreak has occurred). A challenge for the investigator is to achieve a result able to be applied with as low a bias as possible to the general population. In other words, the goal of an epidemiological study of an infectious outbreak is to make the results from a few individuals applicable to the whole population.

A fundamental underpinning of infectious epidemiology is the confirmation that a disease outbreak has occurred. Once this is done, the disease is followed with time. The pattern of appearance of cases of the disease can be tracked by developing what is known as an epidemic curve. This information is vital in distinguishing a natural outbreak from a deliberate and hostile act, for example. In a natural outbreak the number of cases increases over time to a peak, after which the cases subside as immunity develops in the population. A deliberate release of organisms will be evident as a sudden appearance of a large number of cases at the same time.

Tracking diseases with technology. Many illnesses of epidemiological concern are caused by microorganisms. Examples include hemorrhagic fevers such as that caused by the Ebola virus. The determination of the nature of illness outbreaks due to these and other microorganisms involve microbiological and immunological techniques.

Various routes can spread infections (i.e., contact, air borne, insect borne, food and water intake, etc.). Likewise, the route of entry of an infectious microbe can also vary from microbe to microbe.

If an outbreak is recognized early enough, samples of the suspected cause as well as samples from the afflicted (i.e., sputum, feces) can be gathered for analysis. The analysis will depend on the symptoms. For example, in the case of a food poisoning, symptoms such as the rapid development of cramping, nausea with vomiting, and diarrhea after eating a hamburger would be grounds to consider Escherichia coli O157:H7 as the culprit. Analyses would likely include the examination for other known microbes associated with food poisoning (i.e., Salmonella ) in order to save time in identifying the organism.

Analysis can involve the use of conventional laboratory techniques (e.g., use of nonselective and selective growth media to detect bacteria). As well, more recent technological innovations can be employed. An example is the use of antibodies to a known microorganism that are complexed with a fluorescent particle. The binding of the antibody to the microbes can be detected by the examination of a sample using fluorescence microscopy or flow cytometry. Molecular techniques such as the polymerase chain reaction are employed to detect genetic material from a target organism. However, the expense of the techniques such as PCR tends to limit its use to more of a confirmatory role, rather than as an initial tool of an investigation. A considerable research effort is ongoing at U.S. National Laboratories to develop quicker, less expensive, and more portable PCR equipment that can be used by inspectors and investigators.

Another epidemiological tool is the determination of the antibiotic susceptibility and resistance of bacteria.

Such laboratory techniques can be combined with other techniques to provide information related to the spread of an outbreak. For example, microbiological data can be combined with geographic information systems (GIS). GIS information has helped pinpoint the source of outbreaks. In addition to geographic based information, epidemiologists will use information including the weather on the days preceding an outbreak, mass transit travel schedules and schedules of mass-participation events that occurred around the time of an outbreak to try and establish a pattern of movement or behavior to those who have been affected by the outbreak. Use of credit cards and bank debit cards can also help piece together the movements of those who subsequently became infected.

Reconstructing the movements of people is especially important when the outbreak is an infectious disease. The occurrence of the disease over time can yield information as to the source of an outbreak. For example, the appearance of a few cases at first with the number of cases increasing over time to a peak is indicative of a natural outbreak. The number of cases usually begins to subside as the population develops immunity to the infection (e.g., influenza). However, if a large number of cases occur in the same area at the same time, the source of the infection might not be natural. Examples include a food poisoning or a bioterrorist action.

Epidemiologists were among the first scientists to effectively utilize the Internet and email capabilities to effectively communicate regarding disease outbreaks. The International Society for Infectious Diseases sponsors PROMED, the global email based electronic reporting system for outbreaks of emerging infectious diseases and toxins, is open to all sources.

FURTHER READING:

BOOKS:

Trestrail, John H. Forensic Epidemiology. Loue, Sana, 1999.

PERIODICALS:

Epidemiology Program Office, CDC. "CDC's 50th Anniversary: History of CDC." Morbidity and Mortality Weekly Report no. 45 (1996): 52530.

ELECTRONIC:

Centers for Disease Control and Prevention. "About CDC." November 2, 2002. <http://www.cdc.gov/aboutcdc.htm> (28 December 2002).

International Society for Infectious Diseases. ProMED-mail. May, 2003. <http://www.promedmail.org/pls/askus/f?p=2400:1000'>(May 12, 2003).

SEE ALSO

Biological Weapons, Genetic Identification
Bioshield Project
Bioterrorism, Protective Measures
CDC (United States Centers for Disease Control and Prevention)
Communicable Diseases, Isolation, and Quarantine
Public Health Service (PHS), United States
World Health Organization (WHO)

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FARINA, ANTONIO; HOYLE, BRIAN D.. "Epidemiology." Encyclopedia of Espionage, Intelligence, and Security. 2004. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

FARINA, ANTONIO; HOYLE, BRIAN D.. "Epidemiology." Encyclopedia of Espionage, Intelligence, and Security. 2004. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1G2-3403300279.html

FARINA, ANTONIO; HOYLE, BRIAN D.. "Epidemiology." Encyclopedia of Espionage, Intelligence, and Security. 2004. Retrieved May 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3403300279.html

Epidemiology

Epidemiology

Epidemiology is the study of the various factors that influence the occurrence, distribution, prevention, and control of disease, injury, and other health-related events in a defined human population. By the application of various analytical techniques including mathematical analysis of the data, the probable cause of an infectious outbreak can be pinpointed. This connection between epidemiology and infection makes microorganisms an important facet of epidemiology.

Epidemiology and genetics are two distinct disciplines that converge into a new field of human science. Genetic epidemiology, a broad term used for the study of genetics and inheritance of disease, is a science that deals with origin, distribution, and control of disease in groups of related individuals, as well as inherited causes of diseases in populations. In particular, genetic epidemiology focuses on the role of genetic factors and their interaction with environmental factors in the occurrence of disease. This area of epidemiology is also known as molecular epidemiology.

Much information can come from molecular epidemiology even in the exact genetic cause of the malady is not known. For example, the identification of a malady in generations of related people can trace the genetic characteristic, and even help identify the original source of the trait. This approach is commonly referred to as genetic screening. The knowledge of why a particular malady appears in certain people, or why such people are more prone to a microbial infection than other members of the population, can reveal much about the nature of the disease in the absence of the actual gene whose defect causes the disease.

Molecular epidemiology has been used to trace the cause of bacterial, viral, and parasitic diseases. This knowledge is valuable in developing a strategy to prevent further outbreaks of the microbial illness, since the probable source of a disease can be identified.

Furthermore, in the era of the use of biological weapons by individuals, organizations, and governments, epidemiological studies of the effect of exposure to infectious microbes has become more urgently important. Knowledge of the effect of a bioweapon on the battlefield may not extend to the civilian population that might also be secondarily affected by the weapons. Thus, epidemiology is an important tool in identifying and tracing the course of an infection.

The origin of a genetic disease, or the genetic defect that renders someone more susceptible to an infection (e.g., cystic fibrosis), can involve a single gene or can be more complex, involving more than one gene. The ability to sort through the information and the interplay of various environmental and genetic factors to approach an answer to the source of a disease outbreak, for example, requires sophisticated analytical tools and personnel.

Aided by advances in computer technology, scientists develop complex mathematical formulas for the analysis of genetic models, the description of the transmission of the disease, and genetic-environmental interactions. Sophisticated mathematical techniques are now used for assessing classification, diagnosis, prognosis and treatment of many genetic disorders. Strategies of analysis include population study and family study. Population study must be considered as a broad and reliable study with an impact on public health programs. They evaluate the distribution and the determinants of genetic traits. Family study approaches are more specific, and are usually confirmed by other independent observations. By means of several statistical tools, genetic epidemiologic studies evaluate risk factors, inheritance and possible models of inheritance. Different kinds of studies are based upon the number of people who participate and the method of sample collection (i.e., at the time of an outbreak or after an outbreak has occurred). A challenge for the investigator is to achieve a result able to be applied with as low a bias as possible to the general population. In other words, the goal of an epidemiological study of an infectious outbreak is to make the results from a few individuals applicable to the whole population.

Such analytical tools and trained personnel are associated more with the developed world, in the sense that expensive analytical equipment and chemicals, and highly trained personnel are required. However, efforts from the developed world have made such resources available to under-developed regions. For example, the response of agencies such as the World Health Organization to outbreaks of hemorrhagic fevers that occur in underdeveloped regions of Africa can include molecular epidemiologists.

A fundamental underpinning of infectious epidemiology is the confirmation that a disease outbreak has occurred. Once this is done, the disease is followed with time. The pattern of appearance of cases of the disease can be tracked by developing what is known as an epidemic curve. This information is vital in distinguishing a natural outbreak from a deliberate and hostile act, for example. In a natural outbreak the number of cases increases over time to a peak, after which the cases subside as immunity develops in the population. A deliberate release of organisms will be evident as a sudden appearance of a large number of cases at the same time.

Analysis of a proper sample size, as well as study type are techniques belonging to epidemiology and statistics. They were developed in order to produce reliable information from a study regarding the association of genetic and environmental factors. Studies that are more descriptive consider genetic trait frequency, geographic distribution differences, and prevalence of certain conditions in different populations. On the other hand, studies that analyze numerical data consider factors like association, probability of occurrence, inheritance, and identification of specific groups of individuals.

Thus, molecular epidemiology arises from varied scientific disciplines, including genetics, epidemiology, and statistics. The strategies involved in genetic epidemiology encompass population studies and family studies. Sophisticated mathematical tools are now involved, and computer technology is playing a predominant role in the development of the discipline. Multidisciplinary collaboration is crucial to understanding the role of genetic and environmental factors in disease processes.

See also Bacteria and bacterial infection; Genetic identification of microorganisms; History of microbiology; History of public health; Infection control; Public health, current issues; Transmission of pathogens

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Epidemiology

EPIDEMIOLOGY

Epidemiology has been defined as "the study of the distribution and determinants of disease frequency in human populations" (Hennekens and Buring). Based on the underlying tenet that disease does not occur at random, this definition provides a framework for the systematic investigation of health, disability, and illness in human populations. Thus, epidemiology has been identified as the science that forms the basis upon which public health decisions can be made.

In its relatively short history, the field of epidemiology has evolved to reflect the changing nature of the primary causes of morbidity (illness) and mortality (death). The term "epidemiology" was first used to refer to the study of the great "epidemics" of the nineteenth century, such as cholera and diphtheria, in an era when infectious diseases accounted for the overwhelming majority of deaths. Improvements in nutrition, sanitation, housing conditions, and water quality in the early twentieth century resulted in the eradication of many infectious diseases and an increase in life expectancy. Today chronic diseases are emerging as the leading causes of death and disability in developed nations, and there is a shift in the demographic profile toward an increasingly aging population. In turn, the understanding of an epidemic has been broadened to reflect any disease that occurs at a greater frequency than would usually be expected in a specified population or geographic region. It has also been recognized that many chronic conditions have a consistent, endemic presence in a given population or geographic area, such as arthritis among older women. Other conditions are so widespread across a country, or even worldwide, that they are considered pandemic, such as obesity in the Western world.

Quantifying the occurrence of disease and its distribution in relation to the characteristics of person, place, and time falls within the domain of descriptive epidemiology. Measures of disease frequency rely on "count" information, the most basic of which is to add up individuals with a condition. However, for making direct comparisons among different groups it is necessary to determine the denominator, or the source population that the individuals come from. "Incidence" refers to the number of new cases of a condition that develop in a defined population within a specified time interval; it is a useful measure to study disease etiology. "Prevalence" refers to all occurrences of a condition in a defined population at a specific point in time; it is useful for health resource planning. Incidence and prevalence are interrelated in that prevalence is a function of both incidence and duration of a condition. For example, the prevalence of Alzheimer's disease exceeds the incidence of Alzheimer's disease because, once diagnosed, Alzheimer's patients live for many years with the disease. Prevalence is a "snapshot" that captures both newly diagnosed cases and previously diagnosed cases still alive. Institutionalized individuals are often excluded from the numerator (cases) and/or the denominator (persons at risk). Given that institutionalization rises dramatically with age, this poses a unique challenge for the reporting of morbidity rates among older age groups.

Analytic epidemiology

Studying the determinants of disease frequency falls within the domain of analytic epidemiology. The interest in causal relationships determines the type of study designs that epidemiologists employ. An experimental design is rarely appropriate to investigate disease etiology in humans: imagine the implications of randomly assigning individuals to smoke or not to smoke in order to study the effects on lung cancer. Therefore, epidemiologists must rely largely on observational study designs to determine association and to assess causation. The two most common observational study designs are the case-control study, in which individuals with and without the disease of interest are selected and prior exposure history is assessed, and the cohort study, in which individuals with and without the exposure of interest are selected and followed to determine the development of disease.

Relative and attributable risk

Measures of disease association include relative and attributable risk. Relative risk estimates the magnitude of an association between exposure and disease, based on the incidence of disease in the exposed group relative to the unexposed group. A relative risk of 1.0 indicates that there is no association between the exposure and outcome; a relative risk of greater than 1.0 indicates a positive association or increased risk; and a relative risk of less than 1.0 indicates an inverse association, or decreased risk (a protective effect). Incidence usually cannot be calculated in a case-control study, because participants are selected on the basis of disease. In this instance, the odds ratio (the ratio of odds of exposure among the cases to that of the controls) approximates the relative risk. Attributable risk, or risk difference, is the absolute difference in incidence between an exposed and unexposed group. It quantifies the risk of disease in the exposed group attributable to the exposure by removing the risk that would have occurred due to other causes. Expressed differently, attributable risk calculates the number of cases of disease among the exposed that could be eliminated if the exposure were eliminated. This is a useful measure of the public health impact of an exposure, assuming there is a cause-effect relationship. It is not possible to calculate attributable risk for most case-control studies, because incidence cannot be determined.

Susan A. Kirkland

See also Health, Social Factors; Surveys.

BIBLIOGRAPHY

Ebrahim, S., and Kalache, A., eds. Epidemiology in Old Age. London: BMJ Publishing Group, 1996.

Hennekens, C. H., and Buring, J. E. Epidemiology of Medicine. Edited by Sherry L. Mayrent. Boston: Little, Brown, 1987.

Rothman, K. J., and Greenland, S. Modern Epidemiology, 2d ed. Philadelphia, Pa.: Lippincott-Raven, 1998.

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epidemiology

epidemiology The study of diseases that affect large numbers of people. Traditionally, epidemiologists have been concerned primarily with infectious diseases, such as typhoid and influenza, that arise and spread rapidly among the population as epidemics. However, today the discipline also covers noninfectious disorders, such as diabetes, heart disease, and back pain. Typically the distribution of a disease is charted in order to discover patterns that might yield clues about its mode of transmission or the susceptibility of certain groups of people. This in turn may reveal insights about the causes of the disease and possible preventive measures.

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"epidemiology." A Dictionary of Biology. 2004. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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epidemiology

epidemiology, field of medicine concerned with the study of epidemics, outbreaks of disease that affect large numbers of people. Epidemiologists, using sophisticated statistical analyses, field investigations, and complex laboratory techniques, investigate the cause of a disease, its distribution (geographic, ecological, and ethnic), method of spread, and measures for control and prevention. Epidemiological investigations once concentrated on such communicable diseases as tuberculosis, influenza, and cholera, but now also encompass cancer, heart disease, and other diseases affecting large numbers of people.

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epidemiology

epidemiology The analysis of the incidence and spread of disease within populations, with the aim of establishing causality. The modern science of epidemiology is often said to have originated with John Snow's identification of a particular source of drinking water as the cause of the 1849 cholera epidemic in London. More recently, linkages between smoking and lung cancer, between heart disease and certain fats, and between the contraceptive pill and breast cancer, have all been established through epidemiological research.

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GORDON MARSHALL. "epidemiology." A Dictionary of Sociology. 1998. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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epidemiology

ep·i·de·mi·ol·o·gy / ˌepiˌdēmēˈäləjē/ • n. the branch of medicine that deals with the incidence, distribution, and possible control of diseases and other factors relating to health. DERIVATIVES: ep·i·de·mi·o·log·i·cal / -əˈläjikəl/ adj. ep·i·de·mi·ol·o·gist / -jist/ n.

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epidemiology

epidemiology (epi-dee-mi-ol-ŏji) n. the study of the distribution of diseases and determinants of diseases in populations, including all forms of disease that relate to the environment and ways of life.

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epidemiology

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