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