Outbreak names a sudden rise in the incidence of a disease, especially to epidemic or near-epidemic proportions. It can also refer to a sudden increase in number of a harmful organism, as in “an outbreak of locusts.”

The Greek ancestors of epidemic translate roughly as “upon the populace;” in English, epidemic refers to an outbreak that affects many persons (or more persons than expected) within a community, area, or region at a given time.

Although epidemic and pandemic can be used interchangeably, epidemic also has a technical application in which the term describes a disease that affects or tends to affect many individuals within a population, community, or region at the same time.

The similar-looking endemic has a general sense meaning “characteristic of or prevalent in a particular field, area, or environment,” as in “problems endemic to translation” or “the self-indulgence endemic in the film industry,” but it also has a specific technical application, meaning “restricted or peculiar to a locality or region, as in “endemic diseases” or “an endemic species.”

Pandemic is by far the most intense of these three terms. The Greek ancestor of pandemic means “of or belonging to all the people” (pan- meaning “all; every; whole; general” and demos meaning “populace”). The word pandemic names an epidemic of unusual extent or severity, one that occurs over a wide geographic area and affects a high proportion of the population.

What may have been the most devastating pandemic ever, the Spanish influenza of 1918-1919, spread around the world and left more than 20 million dead. (See the Collegiate Encyclopedia entry for Influenza Epidemic of 1918-19 for more on this pandemic.) The twentieth century experienced two more pandemics in 1957 and 1968, and epidemiologists at the Centers for Disease Control and Prevention believe another influenza pandemic is highly likely.