epidemic (adj.)
c.1600, from French épidémique, from épidemié "an epidemic disease," from Medieval Latin epidemia, from Greek epidemia "prevalence of an epidemic disease" (especially the plague), from epi "among, upon" (see epi-) + demos "people, district" (see demotic).
epidemic (n.)
1757, from epidemic (adj.); earlier epideme (see epidemy). An Old English noun for this (persisting in Middle English) was man-cwealm.