Definition of Pandemic

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Pandemic: An epidemic (a sudden outbreak) that becomes very widespread and affects a whole region, a continent, or the world.

By contrast:

• An epidemic affects more than the expected number of cases of disease occurring in a community or region during a given period of time. A sudden severe outbreak within a region or a group as, for example, AIDS in Africa or AIDS in intravenous drug users.
• An endemic is present in a community at all times but in low frequency. An endemic is continuous as in the case of malaria in some areas of the world or as with illicit drugs in some neighborhoods.

The word "pandemic" comes from the Greek "pan-", "all" + "demos", "people or population" = "pandemos" = "all the people." A pandemic affects all (nearly all) of the people. By contrast, "epi-" means "upon." An epidemic is visited upon the people. And "en-" means "in." An endemic is in the people.