In epidemiology, an epidemic epi- meaning "on or in" and demic- meaning "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. Another example includes the infamous Black Plague of the Middle Ages.
Defining an epidemic can be subjective, depending in part on what is "expected". An epidemic may be restricted to one locale (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 may be classified as an "epidemic," while many cases of a common disease (such as the common cold) would not.
The term syndemic refers to interacting epidemics that increase the health burden of affected populations. Social conditions that heighten the health risk of populations (e.g. poverty,