Part of the problem with using the words outbreak, epidemic, and pandemic is that they are occasionally used interchangeably. Let's look at how they are alike and how they differ.

Outbreak names a sudden rise in the incidence of a disease, and although it is sometimes used as a synonym of epidemic, it can be used to distinguish a localized epidemic from a more widespread one.

The Greek ancestor of epidemic translates 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 are close in meaning, pandemic is defined as "an epidemic of unusual extent and severity." Therefore, pandemic is by far the most intense of these three terms. The Greek roots of pandemic mean "of or belonging to all the people" (pan-, meaning "all; every; whole; general," and demos, meaning "populace").

Endemic, in the context of health discussions, is usually used as an adjective that means "restricted or peculiar to a locality or region," as in "endemic diseases." It can be used as a noun in specialized science writing meaning "an endemic organism." But the Unabridged also gives an older sense: "a disease restricted or peculiar to a locality or region." The Greek etymon endemia meant "action of dwelling" (en-, meaning "in" and demos, meaning "populace"). Medically speaking, endemic describes a disease that is constantly present to a greater or lesser extent in a particular place.