In epidemiology, an epidemic (from Greek epi- upon + demos people) occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is "expected," based on recent experience (the number of new cases in the population during a specified period of time is called the "incidence rate"). (An epizootic is the analogous circumstance within an animal population.) In recent usages, the disease is not required to be communicable; examples include cancer or heart disease.
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