The map shows the predicted cholera risk based on analysis and satellite data in Yemen, June 2017. Blue color indicates low risk of cholera while red color indicates high risk of cholera. Image: West Virginia University/Antar Jutla.


An epidemic is generally defined as the occurrence in a community or region of cases of an illness, specific health-related behaviour, 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 (WHO).

Facts and figures

Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts. More specifically, an epidemic may result from:

  • A recent increase in amount or virulence of the agent
  • The recent introduction of the agent into a setting where it has not been before
  • An enhanced mode of transmission so that more susceptible persons are exposed
  • A change in the susceptibility of the host response to the agent, and/or
  • Factors that increase host exposure or involve introduction through new portals of entry (Centers for Disease Control and Prevention)

Epidemics may be the consequence of disasters of another kind, such as tropical storms, floods, earthquakes, droughts, etc. Epidemics may also attack animals, causing local economic disasters (IFRC).

Accordingly to WHO’s classification, these are the major pandemic and epidemic diseases:

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