The term "prevention" encompasses the philosophy, credo, programs, and practices that aim to defer or eliminate diseases, disability, and other forms of human suffering. Additional discussions of disease prevention, the stages of prevention (primary, secondary, and tertiary) and the issues of clinical prevention in the setting of personal health services can be found elsewhere in this encyclopedia. The notion of prevention in populations has a long history of discovering and eliminating the causes of disease. For example, in the 1840s Hungarian physician Ignaz Phillipp Semmelweis reduced the rates of puerperal sepsis among pregnant women through attendant hand washing. In the 1850s, British physician John Snow helped abort an outbreak of cholera in London that was due to a contaminated water supply. The gradual assumption of sanitary practices in public health and preventive activities in clinical practice has been in place for a long time and is increasing, although always challenging and incomplete.
THE CONTEXT OF PREVENTION: THE GENERAL CAUSES OF DISEASE AND DISABILITY
The causes of disease and disability are gradually being discovered and either removed or ameliorated due to scientific advances as well as clinical and preventive interventions. While there are many measures of health, one of the most basic, mortality rates, improved at an unprecedented rate during the twentieth century, providing a strong basis for optimism that new preventives and treatments will continue to enhance health status. However, many diseases and conditions and other causes of human suffering are of unknown or incompletely understood causes. While striving to optimize health status and minimize dysfunction and disability within populations and individuals, it is likely that there will always be a health and functional burden on societies. There are several reasons for this including: uncontrollable acts of nature, such as meteorological and climatic catastrophes; war and other forms of interpersonal violence; unanticipated adverse effects of advancing technology or occupational exposures; adverse effects of health interventions (even if the net health benefit is positive); the constant evolution of infectious organisms; naturally occurring errors in function, which will inevitably occur among complex biological systems, even in the absence of known environmental stimuli; the uncontrollability of individual behavior; and the unintended consequences of health-giving interventions, such as the development of resistance to antibiotics that once successfully cured a wide variety of life-threatening infections.
While public health and the medical sciences continue to develop new preventive and curative modalities, there are ecological factors and forces intimately related to diminished population and individual health that could be addressed even in the absence of clear causal or pathogenetic mechanisms. One of the most important is the close relation between socioeconomic status and health. Both within and among populations, those with higher levels of affluence and various social and economic resources in general have higher levels of health by almost all available indices. One particularly common finding is the relation between income inequality and mortality. There may be several explanations for the generally strong association between income levels and subsequent mortality: (1) higher income levels are a measure of a safer physical environment, including occupational exposures and the general environment; (2) higher income purchases more effective personal and family health services; (3) higher income and wealth levels are literal markers of social status, with lower levels being characterized by increased stress from social oppression and distrust; and (4) income and wealth are markers of increased education and healthy behaviors. It is also possible that the association occurs because individuals with physical or mental conditions have a lesser ability to earn higher levels of income and accumulate wealth.
Even in the absence of full explanations for the income-disease association, some possible solutions may be available, including social policies that limit large levels of income inequality, and expenditures on economic development, which might improve health status secondarily as well as increase access to personal health services. However, the evidence for the effects of these social and economic policies is incomplete, and additional intervention studies are needed.
Despite the presence of global factors that appear to be important forces for disease causation, and the likelihood that disease and disability will quite likely continue to be public health and clinical challenges, a substantial amount is known about the causes of many conditions, and preventive interventions are available to lessen, if not eliminate, their public health burden. When considering disease prevention, it is axiomatic that most important clinical illnesses have multiple causes. For example, deaths from certain viral infections may be caused by the lack of immunization facilities, the failure to handle and administer vaccines properly, household crowding, inadequate nutrition and lack of knowledge to seek early care when the infection appears, as well as biologic variability in susceptibility. Coronary artery disease is caused by several factors, including genetic contributions, high fat diets, cigarette smoking, elevated blood cholesterol levels, and inadequate exercise: Preventive action aimed at several of these factors should have a salient effect. Thus, interventions at several critical points in disease causation—clinical, behavioral, policy, and educational—may all decrease morbidity and mortality. In many instances, multifaceted interventions may provide the best levels of prevention rather than any single approach. A corollary principle is that a preventive intervention at one locus may help prevent several conditions. An effective clinical smoking-prevention program will decrease the risk of several heart and lung diseases.
THE CONTEXT OF PREVENTION: WHERE PREVENTION TAKES PLACE
Prevention takes place at almost all important societal venues, which may be considered in a few major categories (although there is overlap among them): (1) prevention that is facilitated by a healthful environment; (2) prevention optimized by healthful personal behaviors; (3) preventive interventions delivered by health professionals—socalled clinical prevention; and (4) prevention that occurs through social actions, including political, policy, economic, educational, and other group behaviors.
Examples of each preventive venue is instructive. A healthful physical and biological environment is attained when known harmful agents are eliminated, such as lead, automotive exhaust gases, asbestos particles, and viral infectious particles. However, it is very difficult to determine the risks to health of small amounts of certain environmental agents, naturally occurring or contaminants, and risk assessment methods may be employed. Healthful personal behaviors, such as regular, appropriate exercise programs and avoiding risk-taking behaviors, such as tobacco or illicit drug use, will add measurably to positive preventive outcomes. Clinical preventive interventions, such as cancer screening, clinical health counseling, and routine immunizing practices, will add important elements of prevention to individuals and families. Finally, social and administrative activities provide some of the best prevention available in communities. Examples include providing laws that deter underage alcohol and tobacco consumption, health system policies that promote early disease detection, and taxation policies that deter purchase of harmful products such as cigarettes and firearms. All prevention activities work in concert to provide as safe and healthful an environment as possible, but, as noted below, the secondary consequences of each activity should be understood as much as possible.
DISEASE PREVENTION AND HEALTH PROMOTION
Disease prevention is often distinguished from health promotion. While the absence of important conditions is a most worthy goal, it is also useful to consider the attainment of positive health states, where not only are clinical conditions not present, but the highest levels of physical, mental and social well-being are attained. The term "health promotion" has been used to encompass interventions and behaviors that prevent diseases, but many of these same activities can be valuable for attaining the most positive functional performances, emotional attitudes, and states of well-being, irrespective of disease occurrence and risk. Some of this may be obtained by abstinence from tobacco, regular exercise programs, consumption of lower fat diets, and provision of educational opportunities. An additional term often used in public health is "health protection." This term has been used in several contexts, but often encompasses both health promotion and disease prevention, and usually refers to the programmatic and regulatory structures that are designed to limit harmful exposures and enhance health status of particular groups or the general population.
THE POSITIVE AND NEGATIVE EFFECTS OF PREVENTIVE INTERVENTIONS
The goals of prevention would be argued by few. Fundamentally, nearly all would agree that avoiding diseases, disability, and suffering would be beneficial for the health of individuals and the public. However, the methods of prevention, even if based in scientific studies of proven efficacy and effectiveness, can be contentious, and where explicit policy, practice, or programmatic interventions are instituted, there may be substantial fiscal costs to individuals and society, as well as adverse effects and moral or ethical disputes as to the appropriateness of the interventions. It is likely that all major activities and environmental exposures in all societies have both positive and adverse effects on individual and community health, and understanding the trade-offs can be a difficult but necessary dimension of prescribing prevention programs. To elaborate this principle, examples are offered for various elements of prevention:
- Routine childhood vaccines eliminate a substantial amount of disease and death, but occasionally have important adverse effects on the health and well-being of some individuals.
- Many medications that are used for disease prevention, such as those which treat hypertension or hypercholesterolemia, will have predictable and well-established adverse drug reactions that can limit their use.
- Screening for early and asymptomatic diseases will often lead to decreased morbidity and mortality from those diseases, but the screening maneuvers may lead to occasional serious adverse effects, such as perforation of the intestine during colonoscopy. Also, since most screening tests are not perfectly accurate, it is possible that someone might be incorrectly told that a test showed no abnormality when one actually exists—this individual may take inappropriate actions based on this inadvertently false or misleading information.
- Various mechanical devices will enable some disabled individuals to extend their functional range, but the device itself might lead to occasional injuries to that individual or to others assisting him. For example, there is a real and detectable injury rate due to wheelchair use. Such use may be appropriate, but may have incumbent adverse effects.
Thus, it is important for preventive interventions not only to be effective, but also that the cost-effectiveness and benefits of these interventions be understood. Without this, the net health change may not necessarily be positive.
Some preventive interventions are controversial not because they cause some adverse effects or because they don't always work, but rather because they inadvertently promote some level of the behavior or condition they are trying to prevent. This general problem falls under a phenomenon called "harm reduction." While somewhat over-simplified, the following are examples of when and how this occurs:
- The promotion of cigarettes that may confer less exposure to certain carcinogens and other toxins may reduce the risk of some smoking-related conditions, but fail to dissuade some persons to quit smoking altogether because of the perception that the cigarette is "safer," when in fact it may not be very safe and the overall effect is negative.
- Needle exchange programs, which are intended to supply drug addicts with uncontaminated needles that would decrease the risk of blood-borne infections such as AIDS (acquired immunodeficiency syndrome) or hepatitis may be beneficial to those who avail themselves of this program, but may allow more needles to be available to others. How these needles would be used by others is sometimes uncertain. The existing evidence suggests that needle exchange programs do not promote illicit drug use.
- Similarly, the distribution of condoms to demographic or other groups at high risk of sexually transmitted diseases and unwanted pregnancies may be a preventive for some, but others are concerned that this may promote increased sexual activity, with its own health and moral dimensions.
Thus, it is possible that certain preventive interventions are helpful to those who use them, but in theory, the net benefit to the population's health may not be as great as otherwise would be anticipated. The trade-offs and secondary consequences thus should be understood for any preventive maneuver.
PREVENTIVE INTERVENTIONS: UNIVERSAL OR BASED ON SPECIAL RISK
Some preventive interventions are intended for application to high-risk or special-risk individuals. Other inventions are intended for the entire population, such as routine childhood immunizations, various educational programs in primary and secondary educational settings, and the pasteurization of milk for general distribution. Some are obviously only necessary for one gender, such as cervical cancer screening for women or prostate cancer screening for men, and some are intended only for those at risk for the unhealthy exposure of interest, such as antimalarial prophylaxis for those traveling to or residing in areas where such exposure is possible. Most authorities believe that screening infants and children for increased lead exposure, using blood levels, should be reserved for those at increased risk based on environmental exposure characteristics. Risk levels are generally defined by epidemiological studies, although in many instances the evidence necessary for precisely defining risk is often incomplete. For this and other reasons, the threshold for what constitutes "increased" risk for interventions aimed at persons with increased risk does not follow any rule as to how high the risk must be before invoking that intervention. The level chosen may be related to the risk assessment (measurement) methodology, the resources to be expended, and the amount of expected benefit. Other considerations may be of a policy nature, including decisions concerning the alternative public health or preventive uses for the intended resources.
Depending on the preventive intervention, it may be deemed that all persons in a given population are at "high" risk. One important example is coronary artery disease (heart attack, stroke, and related conditions). Here the level of risk is not only stratified within the population, but also contrasts are made with other populations. Within many Western countries, even those at lower risk according to within-population standards may be at much higher risk compared to those in some developing countries where coronary artery disease is much less common. Thus, it may be deemed that all persons in certain Western countries are at "high" risk, leading to universal and more aggressive interventions across a given population. It follows from this principle that some risk levels may be characteristic of populations and not only of individuals within those populations. Public health practice should always take that into account when providing prevention programs.
While the rhetoric defining risk status doesn't lend itself to easy quantification, as previously noted, every society has identified persons by modern public health and epidemiological methods who are at "very high" risk of certain conditions. Examples include persons in certain occupations who are exposed to high levels of environmental toxins or persons with certain genetic characteristics that define very high rates of disease onset. Such situations should be addressed by rigorous preventive interventions where possible, or in the case of genetic conditions, by at least optimally defining and minimizing risk to individuals and families. Other individuals may be at high risk of various conditions by virtue of patterns of risk-taking behaviors, and at least in some instances these can be identified and modified to some extent. Persons at very high risk of conditions, often justifiably, require levels of attention and surveillance far different from others at increased risk as well as other preventive interventions. Whether intensive attention is merited depends on the evidence that disease occurrence and human suffering can be limited.
EPIDEMIOLOGY: THE SCIENTIFIC BASIS FOR PREVENTION
While clinical prevention is aimed at individual patients and patient groups, or at those with special risks, prevention in public health is generally aimed at entire, geographically defined populations. But from a global public health perspective, clinical and population prevention are intimately linked in many ways. Patient groups sustain all of the general exposures and risks of others in a defined community, and thus require the same preventive interventions as those who are not patients. In fact, many important disease-causing factors are characteristics of communities only, and not of individuals, such as ambient air pollution levels or the availability of high-quality fire protection and health education programs in secondary schools. Conversely, many elements of population health promotion and disease prevention, such as behavioral modification programs and immunizations, are performed within the health care system, which is obviously a critical component of population health. Since nearly all citizens of communities are also health care patients at some point in their lives, reconciling the population and patient domains is necessary.
Epidemiology is the science that provides the rationale and quantitative basis for preventive interventions in both patients and communities, and in turn evaluates the effects of those interventions. This discipline describes the health characteristics and status of groups and populations, as well as their trajectories and outcomes, and quantifies the impact of various environmental exposures and personal factors on the occurrence of important health conditions. Epidemiology is largely an observational science, in that it observes disease population occurrence and environmental exposures, deduces causal pathways and mechanisms, and suggests control procedures. But it includes a strong element of randomized trials and other experimental designs where possible, such as in evaluating the efficacy of a vaccine. To do its work, epidemiology draws heavily from many other sciences and disciplines that inform health status and outcomes, such as clinical medicine, demography, behavioral science, microbiology, toxicology, administrative science, genetics, and molecular biology. As noted, this is testimony to the multifactorial nature of disease causation and the need for multidisciplinary approaches to disease prevention and health promotion. An example of cross-disciplinary activity within epidemiological disease control programs is the application of social marketing. This is an approach to communicating and disseminating health information to the community for behavioral change, using the techniques of commercial marketing. Examples of public health campaigns and programs that have used these techniques are promoting condom use among sexually active teenagers and the use of the "designated driver" in an attempt to decrease alcohol-related auto crashes and injuries.
Epidemiology's tasks usually require the calculation of disease rates, which in turn requires both a numerator (accurate disease counts), and a denominator (the population at risk for health change). Critical to both is accuracy. The population denominator, whether whole communities, important demographic segments, or groups of patients in clinical settings, must be understood. For groups defined administratively, such as patients in a hospital or clinical system, record systems will usually furnish adequate counts. For geographically defined populations, an accurate census is critical, and may not always be available. Even within industrialized nations, high levels of population migration or undocumented persons and lack of cooperation with the census in general may lead to population undercounting, often most acute for groups comprising the most important public health constituency. In many instances, effective epidemiological and public health practice requires population counts and assessments more accurate than those obtained by conventional means, employing network and other sampling or estimation techniques.
In addition to accurately determining the size of geographically defined populations, demographic trends in the United States and other industrialized countries are instructive for their effects on disease occurrence and public health. Perhaps the two most important trends are the increase in total population, although not to the extent this is occurring in developing countries, and the "aging" of populations, where the proportion of older persons is increasing relative to other age groups. Increasing population size has clear implications for environmental quality; the availability of basic resources, such as energy, transportation, and water; the transmission of infectious agents; and the nature of social interactions, which can have both negative and positive health effects. An older population similarly will have complex health effects. In general, rates for felonies and sexually transmitted diseases will be lower, but the number of cases of the chronic illnesses of older persons, such as coronary disease and stroke, cancer, diabetes, arthritis, and dementia, will likely increase. Also, since there is a progressively lower proportion of working-age persons, this may put stresses on national and regional economies, and in turn on population health status.
These demographic shifts will change the nature of prevention programs. There may be more emphasis on the prevention and early detection of the chronic conditions of older persons, and somewhat less emphasis on the conditions of younger persons, although all are important. Lower birth rates may shift resources away from maternal and child health programs toward the prevention of disability. The trend toward higher population size, improved survivorship, lower fertility rates, and a higher proportion of elders, the so-called epidemiologic transition, is occurring among many developing countries and over time will most likely shift their disease rates and prevention priorities in a similar direction to that of developed countries.
Determining the numerators for groups and defined populations (counting the diseases, conditions, and other health states in those populations) goes under the general heading of "surveillance." This takes many forms, and knowledge on disease occurrence and health status in many populations is often, at best, incomplete. For example, if a condition is never diagnosed, or if a sick individual doesn't seek medical care, then a clinical event usually remains unknown. Surveillance may be considered in two general categories: active and passive. In the former, information is collected by actively searching for disease occurrence, such as through population surveys, medical record review, and disease marker determinations in population samples. In the latter, reports are accepted from routine reporting and other voluntary sources, irrespective of whether disease reporting is a community regulation or law.
Depending on the condition, different types of surveillance techniques become most important. Historically and currently, communities have designated a set of diseases and conditions that, when medically recognized, should be reported to public health or other medical authorities. Most of these are infectious and communicable conditions, but chronic illnesses are often represented, as can any condition that might be a threat to population health. Many conditions are detected only with appropriate serological or microbiological laboratory techniques, such as many infectious and communicable conditions. Without these techniques, the infections usually would not be precisely identified and control measures executed. Thus, the public health laboratory becomes an indispensable part of a surveillance system. Laboratory ("biomarker") surveillance for some communicable diseases may be routinely performed irrespective of human clinical illness, such as by routine monitoring of sylvatic animals or patients who present to emergency rooms with any relevant clinical syndrome. Some chronic illnesses all require laboratory confirmation, such as accurate histopathology for cancer cases.
Surveillance systems may operate with other techniques, such as clinical record review. While ethical and privacy issues may deter some disease detection, this can be a very important tool for early identification of conditions with public health and preventive import. For those conditions that are not brought to medical attention or for which a diagnosis is not made, the most common approach would be population sample surveys. Here, representative samples would be interviewed or otherwise studied in an attempt to determine the prevalence and incidence of conditions not otherwise detected, or among persons not availing themselves of medical care. Population surveys also afford the opportunity to determine rates of personal behaviors and exposures that are related to disease causation, and further target disease control programs. Special studies on these population samples may reveal physical, mental, dental, or other disorders and form the basis for targeted preventive interventions. The same surveillance systems that define populations at risk and the nature and extent of preventive programs can continue to determine population disease rates as preventive interventions are invoked and applied as part of public health program evaluation.
Robert B. Wallace
(see also: Demographic Transition; Disease Prevention; Epidemiologic Transition; Epidemiology; Notifiable Diseases; Prevention Research; Preventive Health Behavior; Preventive Medicine; Primary Prevention; Registries; Secondary Prevention; Surveillance; Tertiary Prevention )
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