A large-scale outbreak of infectious disease that spreads across populations and regions. It can cause significant morbidity and mortality and affect the economy. It is different from sporadic, which describes diseases that occur infrequently and only when a population has been exposed (e.g. rabies or polio). It is also different from endemic, which describe diseases that are constantly present and not limited to a specific area.
Epidemics have occurred throughout history, and are likely to continue occurring. They are often caused by the emergence of new pathogens into human populations from wildlife reservoirs, or by ecological changes that create conditions favourable for transmission. Natural disasters such as droughts, floods and earthquakes can result in mass displacement of people and facilitate disease transmission. Other factors such as social disruptions like civil war, tourism and climate change can also contribute to epidemics.
The key to controlling an outbreak is to reduce the rate at which people become infected, and thereby the number of cases. This can be done by imposing strict contact restrictions, as was done in the 1918 Influenza pandemic. However, this limits herd immunity and increases the risk of the outbreak flaring up again when the suppression measures are removed. It is therefore important to monitor very carefully and to have contingency plans at a national and local level for the rapid evacuation of populations at risk.
When the number of new infections is plotted over time it typically forms a curve, with an initial steep rise followed by a slower decline. These patterns can be used to predict the outbreak and inform preparedness planning.
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