From Ebola Wiki
An epidemic is, as defined be the Centers for Disease Control and Prevention (CDC), when there is an occurrence of a disease within an area is clearly in excess of the expected level (based on the community baseline) for a given time period.
- "Principles of EPIDEMIOLOGY Second Edition". Author CDC. Pub date Dec 1992. Web 29 Oct 2014 http://www.facmed.unam.mx/deptos/salud/bibliotecav/epi_course.pdf