As an adjective, epidemic means spreading rapidly and extensively by infection and affecting many individuals at the same time, or, in its figurative uses, widely prevalent. As a noun, it refers to a disease (or, figuratively, any other phenomenon) that is epidemic. By definition, epidemic only applies to infectious diseases, so it does not technically apply to, say, obesity, beetles, or laptop theft. People seem to love using epidemic this way, though, so there’s probably nothing we can do about it.
Endemic, too, can be used as both an adjective and a noun. As an adjective, it means prevalent to a particular locality, region, or people (with one synonym being native). As a noun, it means something that is endemic. Writers so often use epidemic to describe things that are endemic that there’s almost no point in fighting it. The oft-discussed “obesity epidemic” in the U.S., for example, is really more endemic than epidemic, yet there’s little chance that the usage will change.
Examples
These writers use epidemic well:
United Nations teams in Haiti believe that the cholera epidemic’s official numbers of 1,800 deaths and nearly 81,000 people infected could be double that because of difficulties in reporting … [NY Times]
Whooping cough—known medically as pertussis—has reached epidemic levels, health officials said. [The Downey Patriot]
New research suggests a hidden epidemic of suicide among younger women with military service. [Medical News Today]
And these writers use endemic well:
It is home to beach pinweed, a vascular plant endemic to the strait, and several other rare and uncommon species. [Winnipeg Free Press (article now offline)]
Some say anti-Semitism in Spain is endemic and will never change … [Haaretz.com]
Malaria is an endemic disease in Yemen and is responsible for 4.9 deaths per 100,000 population per year and 43,000 disability adjusted life years lost. [7th Space]
