In epidemiology, an epidemic is defined as a disease that appears as new cases in a given human population, during a given period of time, at a rate that substantially exceeds “normal,” based on recent experience. However, the definition of epidemic can be subjective depending on what is “normal.” For example, a few cases for a very rare disease like rabies may be classified as an epidemic, while cases of a common disease, as the common cold, would not.

Epidemics can be categorized based on the size or the intensity of appearance of cases of a new disease. For example, an epidemic may be restricted to one locale (an outbreak), more general (an epidemic) or even global (pandemic). Also, when diseases occur at a constant but ...