Infection

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late Middle English: from late Latin infectio(n- ), from Latin inficere ‘dip in, taint’ (see infect).

wiktionary

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From Old French infection, from Vulgar Latin *infectiō.

etymonline

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infection (n.)

late 14c., "infectious disease; contaminated condition;" from Old French infeccion "contamination, poisoning" (13c.) and directly from Late Latin infectionem (nominative infectio) "infection, contagion," noun of action from past participle stem of Latin inficere "to spoil, to stain" (see infect). Meaning "communication of disease by agency of air or water" (distinguished from contagion, which is body-to-body communication), is from 1540s.