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

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


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).

Collins Dictionary of Sociology, 3rd ed. © HarperCollins Publishers 2000
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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 Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The study of the mass aspects of disease.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


the branch of medical science concerned with the occurrence, transmission, and control of epidemic diseases
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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In the United Kingdom (UK, March-July 2006) another outbreak associated with D6-2005 (MVs/Redhill.GBR/10.06) was epidemiologically linked to Italy, although genotype D6 had not been directly detected in this country.
The genotype identified in cases epidemiologically linked to this patient was D4, a genotype currently circulating in France.
* Import-associated cases include 1) internationally imported cases, 2) import-linked cases (those related epidemiologically to imported cases), and 3) imported virus cases (for which no epidemiologic link has been identified but viral genetic evidence indicates an imported measles genotype).
All of the 19 cases were linked epidemiologically, and all but one occurred in children and adolescents aged 9 months to 18 years.
These cases are subclassified into four groups: import linked (i.e., epidemiologically linked to an imported case); imported virus (i.e., cases that cannot be linked epidemiologically to an imported case but for which viral genetic evidence indicates an imported measles genotype); endemic (i.e., cases for which epidemiologic or virologic evidence indicates a chain of measles virus transmission that is continuous for [greater than or equal to] 12 months within the United States); and unknown source (i.e., all other cases acquired in the United States for which an epidemiologic or virologic link to importation or to endemic transmission within the United States cannot be established after a thorough investigation) (3).
coli plasmid donors selected for this study were a small sample of epidemiologically unrelated isolates representative of those carrying a [bla.sub.CMT-2] [beta]-lactamase gene on plasmids classified as type A or B on the basis of the [bla.sub.CMY-2] hybridization pattern (6,13).
In addition, we estimated the proportion of RFLP-defined links that can be explained epidemiologically. Strains were defined as clustered if the RFLP pattern was shared by [greater than or equal to] 2 patients.
Indigenous cases are classified into three groups: import-linked (i.e., epidemiologically linked to an imported case); imported virus (i.e., cases that cannot be linked epidemiologically to an imported case but for which imported virus has been isolated from the patient or from an epidemiologically linked patient); and unknown source (i.e., all other cases acquired in the United States for which no epidemiologic link or virologic evidence indicates importation).
Of the 34 cases, 14 (41%) were laboratory confirmed either by serologic testing that detected measles-specific IgM antibodies, polymerase chain reaction analysis of urine specimens, or both; the other 20 cases were in patients with rash illness who were linked epidemiologically to the confirmed cases.
Measles cases are confirmed either by laboratory test or by meeting the clinical case definition (i.e., illness characterized by generalized maculopapular rash lasting [greater than or equal to] 3 days, a temperature of [greater than or equal to] 101[degrees]F [[greater than or equal to] 38.3[degrees]C], and cough, coryza, or conjunctivitis) and being epidemiologically linked to a laboratory-confirmed case.
Two other persons epidemiologically associated with the index case had compatible symptoms and positive serologic results, and thus can be considered probable case-patients.
The outbreak consisted of 11 laboratory-confirmed cases: nine cases in a boarding school in eastern Pennsylvania and two epidemiologically linked cases in New York City (NYC).

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