ἐπιδήμιος

Epidemic comes from Greek ἐπιδήμιος (epidḗmios), from ἐπί (epí, 'upon, on, at') and δῆμος (dêmos, 'the people, the populace, the district'). The adjective described anything that had come upon or was present among the people of a place, including people themselves: a person who was epidḗmios was at home, present in their own land, as opposed to someone who was apodḗmios (away from their country, abroad). The famous Hippocratic collection of case notes is titled Ἐπιδημίαι (Epidēmíai) — which translators render as 'Epidemics' but which might more literally be 'Things among the people' or 'Local visitations': records of what was happening to the people of particular places at particular times. The title did not necessarily imply catastrophe; it meant clinical observations made in the community.

The Hippocratic Epidemics are among the oldest surviving documents of systematic medical observation. Over seven books, compiled by different hands over perhaps two centuries, they record case after case of individual patients — name, occupation, age, symptoms, course, outcome — alongside sections on the general character of diseases in particular places and seasons. The concept of an epidemic constitution — a distinctive character of disease in a given place and time — was the basis of ancient epidemiology: the recognition that individual illness was related to population patterns, that what struck one person struck many, and that place, season, and air shaped the character of communal disease. The epidḗmios was the disease that had come to visit the people of a community.

Greek and Latin writers applied epidḗmios and its Latin derivative epidemius to the great plagues that struck the ancient Mediterranean world. The Plague of Athens (430–427 BCE), described by Thucydides with clinical precision, was an epidemic in the Hippocratic sense: it came upon the Athenian dêmos with devastating speed and moved through the population without respect for rank or piety. The Antonine Plague of the second century CE and the Plague of Cyprian in the third century were both described in terms that drew on the epidḗmios framework. The word designated not just a disease cluster but a specific relationship between a disease and a population: the disease was a visitor, an arrival from outside the normal order, something that had 'come upon' a community that was its host.

English borrowed 'epidemic' in the late sixteenth century, initially as an adjective applied to diseases that spread widely through a community. By the seventeenth century it was also used as a noun. The technical distinction between epidemic and endemic (habitually present) and pandemic (worldwide) was developed by early modern physicians and public health officials trying to describe different scales of population-level disease. Today, epidemic is precisely defined in epidemiology: a disease occurrence clearly in excess of normal expectancy in a defined community, area, or season. But in ordinary language, it has been extended to social phenomena — an epidemic of gun violence, an epidemic of loneliness, an epidemic of misinformation — preserving the Greek structure of something that has 'come upon the people' in numbers that exceed the normal.