early 17th century (as an adjective): from French épidémique, from épidémie, via late Latin from Greek epidēmia 'prevalence of disease', from epidēmios 'prevalent', from epi 'upon' + dēmos 'the people'
A disease that quickly and severely affects a large number of people and then subsides is an epidemic: throughout the Middle Ages, successive epidemics of the plague killed millions. Epidemic is also used as an adjective: she studied the causes of epidemic cholera. A disease that is continually present in an area and affects a relatively small number of people is endemic: malaria is endemic in (or to) hot, moist climates. A pandemic is a widespread epidemic that may affect entire continents or even the world: the pandemic of 1918 ushered in a period of frequent epidemics of gradually diminishing severity. Thus, from an epidemiologist’s point of view, the Black Death in Europe and AIDS in sub-Saharan Africa are pandemics rather than epidemics.