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    Epidemiology is the study of the distribution and determinants of diseases within human populations. Research in this field is based primarily upon observing people directly in their natural environments.
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    Epidemiology can be used for descriptive purposes, such as surveillance of the occurrence (incidence) of a particular illness.
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    Epidemiology can be used for analytic purposes, such as studying risk factors for disease development.
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    Epidemiologic methods can be used to assess the performance of diagnostic tests.
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    Epidemiology can be used to study the progression or natural history of a disease.
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    Epidemiologic methods can be used to study prognostic factors, which are determinants of the progression of a disease.
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    Epidemiology can be used to evaluate treatments for a disease.

imageA 29-year-old previously healthy man was referred to the University of California at Los Angeles (UCLA) Medical Center with a history of fever, fatigue, lymph node enlargement, and weight loss of almost 25 lb over the preceding 8 months. He had a temperature of 39.5°C, appeared physically wasted, and had swollen lymph nodes. Laboratory evaluation revealed a depressed level of peripheral blood lymphocytes. The patient suffered from simultaneous infections involving Candida albicans in his upper digestive tract, cytomegalovirus in his urinary tract, and Pneumocystis carinii in his lungs. Although antibiotic therapy was administered, the patient remained severely ill.

Epidemiology is a fundamental medical science that focuses on the distribution and determinants of disease frequency in human populations. Specifically, epidemiologists examine patterns of illness in the population and then try to determine why certain groups or individuals develop a particular disease whereas others do not.

Knowledge about who is likely to develop a particular disease and under what circumstances they are likely to develop it is central to the daily practice of medicine and to efforts to improve the health of the public. To prevent an illness, health care providers must be able both to identify persons who because of personal characteristics or their environment are at high risk, and to intervene to reduce that risk. This type of knowledge emerges in many cases from epidemiologic research.

This book serves as an introduction to epidemiologic methods and the ways in which these methods can be used to answer key medical and public health questions. This chapter begins by considering a particular disease, as described in the Patient Profile. Focusing attention on one disease enables us to demonstrate the important contribution of epidemiology to current knowledge about this condition. Although the emphasis is on a single disease, it should be recognized that epidemiologic methods can be applied to a wide spectrum of conditions, ranging from acute illnesses, such as outbreaks of food-borne infections, to long-term debilitating conditions, such as Alzheimer’s disease.

The man in the Patient Profile was referred to the UCLA Medical Center in June 1981. At the time, there was no obvious explanation as to why a healthy young man would suddenly develop concurrent infections ...

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