Occurrence of a particular disease in a large number of people in a particular area. see outbreak, pandemic.

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A sudden increase in the number of cases of a disease in a particular geographic area, beyond the number health officials typically expect. An increase that occurs in a relatively small geographic area or among a small group of people may be called an "outbreak."

For example, the CDC calls HIV/AIDS, which affects 1.2 million people in the United States, an "epidemic." By contrast, the CDC called two cases of sickness from drinking raw milk (listeriosis) in the United States an "outbreak."

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An epidemic is a situation where a disease spreads rapidly among many people, and in a higher concentration than normal. It is on a smaller scale, however, than a pandemic. The global COVID-19 outbreak is considered to be an epidemic, though there are fears among governments and medical communities that it could become a pandemic.

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A large outbreak of disease, taking place over a short period of time. An epidemic might infect a region or a country. Epidemics usually happen when a new disease emerges or when something happens to make people less immune to a disease. A pandemic is an epidemic that spreads to multiple large regions, like several continents or countries.

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An epidemic is a significant, often sudden increase in the number of cases of an infectious disease that rises above what's usually expected for a population in a certain geographic area. (Endemic refers to the baseline or typical level of the disease in that area.)

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A cluster of outbreaks that have spread from one geographical area to others; also see related terms, “pandemic” and “outbreak.”

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The widespread outbreak of an infectious disease. Past epidemics include the 2009 swine flu epidemic, the 2014 Ebola epidemic, and the 2019 measles outbreak in the Democratic Republic of Congo.

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When a virus has spread widely in a local area or region. By contrast, a pandemic indicates global spread.