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field of medicine concerned with the study of epidemicsepidemic,
outbreak of disease that affects a much greater number of people than is usual for the locality or that spreads to regions where it is ordinarily not present. A disease that tends to be restricted to a particular region (endemic disease) can become epidemic if
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, 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 tuberculosistuberculosis
(TB), contagious, wasting disease caused by any of several mycobacteria. The most common form of the disease is tuberculosis of the lungs (pulmonary consumption, or phthisis), but the intestines, bones and joints, the skin, and the genitourinary, lymphatic, and
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, influenzainfluenza
or flu,
acute, highly contagious disease caused by a RNA virus (family Orthomyxoviridae); formerly known as the grippe. There are three types of the virus, designated A, B, and C, but only types A and B cause more serious contagious infections.
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, and choleracholera
or Asiatic cholera,
acute infectious disease caused by strains of the bacterium Vibrio cholerae that have been infected by bacteriophages. The bacteria, which are found in fecal-contaminated food and water and in raw or undercooked seafood, produce a
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, but now also encompass cancercancer,
in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth.
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, heart diseaseheart disease,
any of several abnormalities of the heart and its function in maintaining blood circulation. Heart disease is the cause of approximately half the deaths in the United States each year.
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, and other diseases affecting large numbers of people.


The study of the distribution of diseases in populations and of factors that influence the occurrence of disease. Epidemiology examines epidemic (excess) and endemic (always present) diseases; it is based on the observation that most diseases do not occur randomly, but are related to environmental and personal characteristics that vary by place, time, and subgroup of the population. The epidemiologist attempts to determine who is prone to a particular disease; where risk of the disease is highest; when the disease is most likely to occur and its trends over time; what exposure its victims have in common; how much the risk is increased through exposure; and how many cases of the disease could be avoided by eliminating the exposure.

In the course of history, the epidemiologic approach has helped to explain the transmission of communicable diseases, such as cholera and measles, by discovering what exposures or host factors were shared by individuals who became sick. Modern epidemiologists have contributed to an understanding of factors that influence the risk of chronic diseases, particularly cardiovascular diseases and cancer, which account for most deaths in developed countries today. Epidemiology has established the causal association of cigarette smoking with heart disease; shown that acquired immune deficiency syndrome (AIDS) is associated with certain sexual practices; linked menopausal estrogen use to increased risk of endometrial cancer but to decreased risk of osteoporosis; and demonstrated the value of mammography in reducing breast cancer mortality. By identifying personal characteristics and environmental exposures that increase the risk of disease, epidemiologists provide crucial input to risk assessments and contribute to the formulation of public health policy.

Epidemiologic studies, based mainly on human subjects, have the advantage of producing results relevant to people, but the disadvantage of not always allowing perfect control of study conditions. For ethical and practical reasons, many questions cannot be addressed by experimental studies in humans and for which observational studies (or experimental studies using laboratory animals or biomedical models) must suffice. Still, there are circumstances in which experimental studies on human subjects are appropriate, for example, when a new drug or surgical procedure appears promising and the potential benefits outweigh known or suspected risks. See Disease, Epidemic

Descriptive epidemiologic studies provide information about the occurrence of disease in a population or its subgroups and trends in the frequency of disease over time. Data sources include death certificates, special disease registries, surveys, and population censuses; the most common measures of disease occurrence are (1) mortality (number of deaths yearly per 1000 of population at risk); (2) incidence (number of new cases yearly per 100,000 of population at risk); and (3) prevalence (number of existing cases at a given time per 100 of population at risk). Descriptive measures are useful for identifying populations and subgroups at high and low risk of disease and for monitoring time trends for specific diseases. They provide the leads for analytic studies designed to investigate factors responsible for such disease profiles.

Analytic epidemiologic studies seek to identify specific factors that increase or decrease the risk of disease and to quantify the associated risk. In observational studies, the researcher does not alter the behavior or exposure of the study subjects, but observes them to learn whether those exposed to different factors differ in disease rates. Alternatively, the researcher attempts to learn what factors distinguish people who have developed a particular disease from those who have not. In experimental studies, the investigator alters the behavior, exposure, or treatment of people to determine the impact of the intervention on the disease. Usually two groups are studied, one that experiences the intervention (the experimental group) and one that does not (the control group). Outcome measures include incidence, mortality, and survival rates in both the intervention and control groups.


the study of the incidence and distribution of MORBIDITY and mortality in order to identify the role of nonbiological factors in sickness and health, for example, the relative significance of occupation, geographical location, class, gender, ethnicity, consumption patterns and lifestyle.

Originally the study of plagues and their relationship to population growth and economic development, the method was also called medical mapping when it was being developed in the 19th-century, at the same time as the sanitary reform movement. It provided evidence that the control of illness could be effected by public health measures. Although epidemiological research is informed by medical frames of reference, it is heavily used by sociologists interested in the social distribution of sickness (see BLACK REPORT).



the science that studies the causes, developmental patterns, and extensive spread of certain diseases and the methods that can prevent or control them. Epidemiology evolved as a scientific discipline concerned with infectious diseases, because the diseases often became pandemics that killed millions of people (for example, plague in the sixth and 14th centuries and cholera in the 19th century).

Even the ancients were familiar with various means of controlling epidemics, including removal of the sick from a city, burning the possessions of the sick and dead (in Assyria and Babylon), and using convalescents to care for the sick (Greece). Quarantine was instituted in Europe in the 14th century; lepers were forbidden to use wells or to enter churches or bakeries. In Russia the sick were isolated from the healthy. It was forbidden (1510) to visit the sick or perform burial rites for those who died of plague, and plague victims were buried in separate cemeteries. Gates and barriers of felled trees (1552), including frontier posts (1602), were erected in many places.

The foundations of epidemiology as a science were laid in the 16th century by Fracastoro’s theory of contagious diseases. D. Samoilovich’s research in 18th-century Russia was of major importance. The studies of L. Pasteur, R. Koch, E. Metchnikoff, and others and the discovery of the causative agents of many infectious diseases in the late 19th and early 20th centuries paved the way for objective research and development of epidemiology as a scientific discipline. The first independent university department of epidemiology was established by D. K. Zabolotnyi (author of the first Soviet handbook of epidemiology, 1927) in 1920 in Odessa. Zabolotnyi advanced the theory of the natural endemism of plague. Other pioneers in epidemiology in the USSR were L. V. Gromashevskii (student of Zabolotnyi and author of the theory of mechanical transmission of infection), E. N. Pavlovskii, and K. I. Skriabin.

The doctrine of the epidemic process is the basis of the modern epidemiology of infectious diseases. The elements, or conditions, making up the doctrine are (1) the source of the causative agent of the infection, (2) the mechanism of transmission, (3) the susceptibility of the group, and (4) the patterns of distribution of the disease in relation to social and other environmental factors. Epidemiological methods include epidemiological survey; microbiological, sanitary, and other investigations; comparative and historical study of epidemics; statistical analysis; and experimentation. Advances in epidemiology depend, therefore, on discoveries in microbiology, virology, parasitology, immunology, hygiene, and clinical research. Advances in the creation of vaccines, in disinfection, and in laboratory diagnosis are also applied in epidemiological practice.

In the USSR the epidemiology of infectious diseases is studied at the Central Institute of Epidemiology of the Ministry of Health of the USSR and at the N. F. Gamaleia Institute of Epidemiology and Microbiology of the Academy of Medical Sciences of the USSR (Moscow). The discipline is also studied at institutes of virology, parasitology, vaccines and sera, and natural-endemic infections, as well as in the epidemiology departments of medical institutes and institutes for postgraduate medicine. The Zhurnal mikrobiologii, epidemiologii i immunobiologii has been published since 1924. The leading research centers outside the USSR are the Pasteur Institute in Paris, the Robert Koch Institute in West Berlin, the Lister Institute of Preventative Medicine in London, and the National Institute of Allergy and Infectious Diseases in Bethesda, Md. The American Journal of Epidemiology has been published in Baltimore since 1921.

The concerns of modern epidemiology have extended beyond infectious diseases because of the changing nature of pathology in economically developed countries. The massive, or “epidemic,” spread of cardiovascular, oncological, neurological, mental, and other diseases requires application of the epidemiological method in the study of patterns of distribution and methods of prevention and control. Thus, the term “epidemiology” has been adopted for designating the corresponding branches of cardiology, oncology, psychiatry, endocrinology, and so on.


Zabolotnyi, D. K. Osnovy epidemiologii, vol. 1. Moscow-Leningrad, 1927.
Mekhanizm peredachi infeklsii, 2nd ed. Kiev, 1962.
Baroian, O. V. Ocherki po mirovomu rasproslraneniiu vazhneishikh zaraznykh boleznei. Moscow, 1962.
Epidemiologiia. Leningrad, 1973.
Metelitsa, V. I., and N. A. Mazur. Epidemiologiia i profilaktika ishemicheskoi bolezniserdlsa. Moscow, 1976.



The study of the mass aspects of disease.


the branch of medical science concerned with the occurrence, transmission, and control of epidemic diseases
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