Epidemic

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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’.

wiktionary

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From French épidémique, from épidémie, from Latin epidemia, from Ancient Greek ἐπιδήμιος(epidḗmios), from ἐπί(epí, “upon”) + δῆμος(dêmos, “people”). Surface analysis epi-(“on”) +‎ demic(“of the people”).

etymonline

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epidemic (adj.)

c. 1600, "common to or affecting a whole people," originally and usually, though not etymologically, in reference to diseases, from French épidémique, from épidemié "an epidemic disease," from Medieval Latin epidemia, from Greek epidemia "a stay in a place; prevalence of an epidemic disease" (especially the plague), from epi "among, upon" (see epi-) + dēmos "people, district" (see demotic). Also see -ic.

epidemic (n.)

1757, "an epidemic disease, a temporary prevalence of a disease throughout a community," from epidemic (adj.); earlier epideme (see epidemy). An Old English noun for this (persisting in Middle English) was man-cwealm.