Epidemic
In epidemiology, an epidemic (from Greek epi- upon + demos people) is a disease that appears as new cases in a given human population, during a given 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.)
Defining an epidemic can be sugestive, depending in part on what is "expected". An epidemic may be restricted to one local (an outbreak), more general (an "epidemic") or even global (pandemic). Because it is based on what is "expected" or thought normal, a few cases of a very rare disease like rabies may be classified as an "epidemic", while many cases of a common disease (like the common cold) would not.
Common diseases that occur at a constant but relatively high rate in the population are said to be "endemic". An example of an endemic disease is malaria in some parts of Africa (for example, Liberia) in which a large portion of the population is expected to get malaria at some point in their lifetimes.
Famous examples of epidemics include the bulbonic plague epidemic of Medieval Europe known as the Back Death, and the Great Influenza Pandemic concurring with the end of World War I.
Non-biological usage
The term is often used in a non-biological sense to refer to widespread and growing societal problems, for example, in discussions of mental illness or drug addiction.