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.
Epidemics of infectious disease are generally caused by several factors including a change in the ecology of the host population , a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population .
An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic. A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease would not.
