Epidemic

In epidemiology, an epidemic (επι (epi)- meaning "upon or above" and δεμος (demos)- 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. Epidemiologists often consider the term outbreak to be synonymous to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics

An epidemic may be restricted to one locale, however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic. The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline. 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.

Causes of epidemics
There are several changes that may occur in an infectious agent that may trigger an epidemic these include:
 * Increased virulence
 * Introduction into a novel setting
 * Changes in host susceptibility to the infectious agent
 * Changes in host exposure to the infectious agent

An epidemic disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity epidemic, among others.

Types of epidemics
In a common source outbreak, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed a point source outbreak. If the exposure was continuous or variable, it can be termed a continuous outbreak or intermittent outbreak, respectively.
 * Common source outbreak

In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.
 * Propagated outbreak

Many epidemics will have characteristics of both common source and propagated outbreaks. For example, secondary person-to-person spread may occur after an common source exposure or a environmental vectors may spread a zoonotic diseases agent.

Etymology
The term epidemic derives from a term first attributed to Homer's Odyssey, which later took its medical meaning from a treatise by Hippocrates, Epidemics. Prior to Hippocrates, epidemios, epidemeo, epidamos and other variants had meanings similar to the current definitions of "indigenous" or "endemic". Thucydides's description of the Plague of Athens is considered one of the earliest accounts of a disease epidemic.