<P> An epidemic (from Greek ἐπί epi "upon or above" and δῆμος demos "people") is the rapid spread of infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less . For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic . </P> <P> Epidemics of infectious disease are generally caused by several factors including a change in the ecology of the host population (e.g. increased stress or increase in the density of a vector species), a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population (by movement of pathogen or host). Generally, an epidemic occurs when host immunity to either an established pathogen or newly emerging novel pathogen is suddenly reduced below that found in the endemic equilibrium and the transmission threshold is exceeded . </P>

A disease that is usually present at a low incidence in a population