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.

Epidemiology

Epidemiology is the study of the occurrence, frequency, and distribution of diseases in a given population. As part of this study, epidemiologists—scientists 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 placebo—a "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 variable—antibiotic treatment—made 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 rate—the 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 epidemic—one 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).

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): 525–30.

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)

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

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.

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.

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.

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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