Department of Epidemiology

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What is Epidemiology?


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Public Health is a blend of sciences, skills and convictions that is focused on the preservation and improvement of the health of all people through preventive (rather than curative) measures.

Epidemiology is considered a basic science of public health. Epidemiology is: a) a quantitative discipline built on a working knowledge of probability, statistics, and sound research methods; b) a method of causal reasoning based on developing and testing hypotheses pertaining to occurrence and prevention of morbidity and mortality; and c) a tool for public health action to promote and protect the public's health based on science, causal reasoning, and a dose of practical common sense (1).

The word epidemiology comes from the Greek words epi, meaning "on or upon," demos, meaning "people," and logos, meaning "the study of." Many definitions have been proposed; here are two that capture the underlying principles and the public health spirit of epidemiology:

“Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.” (2).

“Epidemiology is the study of the distribution and determinants of disease frequency in man.” (3).

These definitions of epidemiology include several terms which reflect some of the important principles of the discipline:

Study - Epidemiology is a scientific discipline and has at its foundation, sound methods of scientific inquiry.

Distribution - Epidemiology is concerned with the frequency and pattern of health events in a population. Frequency includes not only the number of such events in a population, but also the rate or risk of disease in the population.

Pattern refers to the occurrence of health-related events by time, place, and personal characteristics.

  • Time characteristics include annual occurrence, seasonal occurrence, and daily or even hourly occurrence.
  • Place characteristics include geographic variation, urban-rural differences, and location of work sites or schools, for example.
  • Personal characteristics include demographic factors such as age, race, sex, marital status, and socioeconomic status, as well as behaviors ( such as occupation or risk-taking activity) resulting in environmental exposures.

This characterization of the distribution of health-related states or events is one broad aspect of epidemiology called descriptive epidemiology. Descriptive epidemiology provides the What, Who, When and Where of health-related events.

Determinants - Epidemiology is often used to search for causes and other factors that influence the occurrence of health-related events such as diseases, syndromes, and injuries. Analytic epidemiology attempts to provide the Why and How of such events by comparing groups with different rates of disease occurrence and with differences in demographic characteristics, genetic or immunologic make-up, behaviors, environmental exposures, and other so-called potential risk factors. Under ideal circumstances, epidemiologic findings provide sufficient evidence to direct swift and effective public health control and prevention measures.

Health-related states or events - Originally, epidemiology was concerned with epidemics of communicable diseases. The discipline was extended to endemic communicable diseases and noncommunicable infectious diseases. Modern epidemiology has been applied to chronic diseases, injuries, birth defects, maternal-child health, occupational health, and environmental health. Now, even behaviors related to health and well-being (amount of exercise, seat-belt use, etc.) are recognized as valid subjects for applying epidemiologic methods. The term "disease" refers to the range of health-related states or events.

Specified populations - Although epidemiologists and physicians in clinical practice are both concerned with disease and the control of disease, they differ greatly in how they view “the patient.” Clinicians are concerned with the health of an individual; epidemiologists are concerned with the collective health of the people in a community or other area. When faced with a patient with diarrheal disease, for example, the clinician and the epidemiologist have different responsibilities. Although both are interested in establishing the correct diagnosis, the clinician usually focuses on treating and caring for the individual. The epidemiologist focuses on the exposure (action or source that caused the illness), the number of other persons who may have been similarly exposed, the potential for further spread in the community, and interventions to prevent additional cases or recurrences.

Application - Epidemiology is more than "the study of." As a discipline within public health, epidemiology provides data for directing public health action. To treat a patient, a clinician must call upon experience and creativity as well as scientific knowledge. Similarly, an epidemiologist uses the scientific methods of descriptive and analytic epidemiology in “diagnosing” the health of a community, but also must call upon experience and creativity when planning how to control and prevent disease in the community. As a public health discipline, epidemiology is instilled with the spirit that epidemiologic information should be used to promote and protect the public's health. Hence, epidemiology involves both science and public health practice. The term applied epidemiology is sometimes used to describe the application or practice of epidemiology to address public health issues.

Examples of applied epidemiology include the following:

  • the monitoring of reports of communicable diseases in the community
  • the study of whether a particular dietary component influences your risk of developing cancer
  • evaluation of the effectiveness and impact of a cholesterol awareness program through quasi-experimental study design
  • analysis of historical trends and current data to project future public health resource needs
  • clinical trial randomizing communities into different strategies for risk reduction

Epidemiology and the information generated by epidemiologic methods have many uses which include:

Population or community health assessment - To set policy and plan programs, public health officials must assess the health of the population or community they serve and must determine whether health services are available, accessible, effective, and efficient. To do this, they must find answers to many questions: What are the actual and potential health problems in the community? Where are they? Who is at risk? Which problems are declining over time? Which ones are increasing or have the potential to increase? How do these patterns relate to the level and distribution of services available? The methods of descriptive and analytic epidemiology provide ways to answer these and other questions. With answers provided through the application of epidemiology, communities and officials can make informed decisions that will lead to improved health for the population.

Individual decisions - People may not realize that they use epidemiologic information in their daily decisions. When they decide to stop smoking, take the stairs instead of the elevator, order a salad instead of a cheeseburger, or choose one method of contraception instead of another, they may be influenced, consciously or unconsciously, by epidemiologists’ assessments of risk. Since World War II, epidemiologists have provided information related to all these decisions. In the 1950s, epidemiologists documented the increased risk of lung cancer among smokers; in the 1960s and 1970s, epidemiologists noted a variety of benefits and risks associated with different methods of birth control; in the mid-1980's, epidemiologists identified the increased risk of human immunodeficiency virus (HIV) infection associated with certain sexual and drug-related behaviors; and epidemiologists continue to document the role of exercise and proper diet in reducing the risk of heart disease. Hundreds of epidemiologic findings are directly relevant to the choices that people make every day, choices that affect their health over a lifetime.

Completing the clinical picture - When studying a disease outbreak, epidemiologists depend on clinical physicians and laboratory scientists for the proper diagnosis of individual patients. But epidemiologists also contribute to physicians' understanding of the clinical picture and natural history of disease. For example, in late 1989 three patients in New Mexico were diagnosed as having myalgia (severe muscle pains in chest or abdomen) and unexplained eosinophilia (an increase in the number of one type of white blood cell). Their physician could not identify the cause of their symptoms, or put a name to the disorder. With considerable success, epidemiologists found other cases of eosinophilia-myalgia syndrome, more fully describing the illness, its complications, and its rate of mortality. Similarly, epidemiologists have documented the course of HIV infection, from the initial exposure to the development of a wide variety of clinical syndromes that include the originally described acquired immunodeficiency syndrome (AIDS). They have also documented the numerous conditions that are associated with cigarette smoking - from pulmonary and heart disease to lung and cervical cancer.

Search for causes - Much of epidemiologic research is devoted to a search for causes, factors that influence one's risk of disease. Often the goal is to identify a cause so that appropriate public health action might be taken. While epidemiology can not necessarily prove a causal relationship between an exposure and a disease, it often provides enough information to support effective action. Examples include John Snow’s removal of the pump handle in nineteenth century London to combat cholera and the withdrawal of a specific brand of tampon that was linked by epidemiologists to toxic shock syndrome in the 1970s-1980s. Just as often, epidemiology and laboratory science converge to provide the evidence needed to establish causation. For example, a team of epidemiologists were able to identify a variety of risk factors during an outbreak of a pneumonia among persons attending the American Legion Convention in Philadelphia in 1976. However, the outbreak was not “solved” until the Legionnaires’ bacillus was identified in the laboratory almost 6 months later.

The faculty in the Department of Epidemiology will work with you to build your knowledge base and technical skills in the field of epidemiology. While the topic areas that we work in are diverse (e.g., cancer, occupational, infectious, heart, genetic, environmental, dental, injury, children and women’s health), we share a common methodology in the field of epidemiology.


  1. Cates WJ. Epidemiology: Applying principles to clinical practice. Contemp Ob/Gyn 1982; 20:147-161.
  2. Last JM, ed. Dictionary of Epidemiology, Third edition. New York: Oxford U. Press, 1995:55.
  3. MacMahon B., Pugh TF. Epidemiology: principles and methods. Boston: Little Brown and Company, 1970:1.
NOTE: This section has been abstracted and modified from Principles of Epidemiology, Second edition the Centers for Disease Control and Prevention Self-Study Course 3030-G.


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