[from the Greek epi (upon), dēmos (people)] The occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy. The community or region and the period in which the cases occur must be specified precisely. The number of cases indicating the presence of an epidemic varies according to the agent, size, and type of population exposed; previous experience or lack of exposure to the disease; and time and place of occurrence. Epidemicity is thus relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year.

A single case of a Communicable Disease long absent from a population or first invasion by a disease not previously recognized in that area requires immediate reporting and (p. 94) full field investigation; two cases of such a disease associated in time and place may be sufficient evidence to be considered an epidemic. Classic epidemics initially identified following the occurrence of small numbers of cases include the epidemic of vaginal cancer in daughters of women who took diethylstilbestrol during pregnancy,³⁷⁸ and the Pandemic of AIDS, which was heralded by a report³⁷⁹ of cases of Pneumocystis carinii pneumonia among gay men in Los Angeles in 1981.

The purpose of Surveillance systems is to identify epidemics as early as possible so that effective control measures can be put in place. This remains a most important task for epidemiology. The word may be used also to describe outbreaks of disease in animal or plant populations.⁴⁸ See also epizootic; epornithic.