The occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy.
The community or region and the period in which the cases occur are specified precisely. The number of cases indicating the presence of an epidemic varies according to the agent, size, and type of population exposed, previous experience or lack of exposure to the disease, and time and place of occurrence.

Epidemic threshold: is the critical number or density of susceptible hosts required for an epidemic to occur; it is used to confirm the emergence of an epidemic so as to increase appropriate control measures.

Adapted from World Health Organization. Definitions: emergencies. Internet. Accessed on June 16, 2016.