Epidemy is a noun used in the medical sense to refer to an epidemic disease. An epidemic disease on the other hand means an outbreak or an exceptionally high incidence of a disease seen in a population or area. An epidemic means affecting or inclined to affect an excessively large number of persons within a population, region or community, or at the same time. It could refer to something that is excessively prevalent.

An epidemic could either be contained in one local (where it is an outbreak), or it could me more general (where it is known as an "epidemic") or it could even be global (usually called a pandemic).

There is a term called the 'epidemiologic triangle' which is used to illustrate the intersection of the three main factors – that it, the Host, Agent, and Environment when it comes to analyzing an outbreak.