As an adjective, epidemic describes diseases and conditions that spread rapidly and extensively by infection and affect many people at the same time, and it’s used figuratively to describe widely prevalent things other than infectious diseases. It also doubles as a noun referring to things that are epidemic. By its scientific definition, epidemic only applies to infectious diseases and not to, say, obesity, beetles, or laptop theft, but such figurative, nonscientific use is common.
Endemic, too, can be both an adjective and a noun. As an adjective, it means prevalent in a particular region or among a people. As a noun, it means something that is endemic. There is sometimes common ground between epidemics and endemics when the words refer to diseases or conditions. For instance, in the first example below, the wave of cholera in Haiti is certainly an epidemic because it spreads rapidly and extensively by infection, but it’s also endemic because it’s mostly confined to Haiti. But while endemic logically applies here, that word is more often used in reference to things that are chronically prevalent among a people or in a region. Epidemics tend to be short-term phenomena and are not chronically prevalent; they run their course and fade away.
Meanwhile, endemic also means native to a given region. It might be used to describe, for example, a plant that lives exclusively in the Sahara desert, or a fish that lives only in the Great Barrier Reef. The word is used this way in the second example below.
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]
Of Africa’s 10 endemic mainland bird families, eight are represented in Ethiopia. [Ethiopia and Eritrea]
Total mortality in the epidemic may have been as high as one-third of the population of Japan, with some areas experiencing a staggering mortality rate of 60 percent. [Epidemics and Pandemics]
If he insists on picking hotels where that type of poolside behaviour is endemic, the rest of us will only chuckle at his discomfort. [letter to Financial Times]
New research suggests a hidden epidemic of suicide among younger women with military service. [Medical News Today]
Qidong City, China, has had high liver cancer incidence from endemic hepatitis B virus (HBV) infection and dietary exposure to aflatoxin. [Carcinogenesis]