epidemic

From French épidémique, from épidémie, from Medieval Latin epidēmia (a reanalysis of the Late Latin plural epidēmia), from Greek ἐπιδήμιος, itself from ἐπί ("upon") plus δῆμος ("people").

The word's very form comes from a mix-up — a Late Latin plural got reanalyzed as a singular noun in Medieval Latin, and that reshaped version is what passed into French and then English.

It shares its root δῆμος ("people") with words built on -demic, since by surface analysis epidemic breaks down as epi- ("on") + demic ("of the people").