In epidemiology, an epidemic (from Greek epi- upon + demos people) is a classification of a disease that appears as new cases in a given human population, during a given menistral period, at a rate that substantially exceeds 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 same thing but for an animal population.) The disease may or may not be contagious.