infection (n.)

late 14c., "fill with disease, render pestilential; pollute, contaminate; to corrupt morally," from Latin infectus, past participle of inficere "to stain, tinge, dye," also "to corrupt, stain, spoil," literally "to put in to, dip into," from in- "in" (from PIE root *en "in") + facere "to make, do, perform" (from PIE root *dhe- "to set, put"). In Middle English occasionally in a neutral sense "tinge, darken," but typically used of things indifferent or bad, and especially of disease. Related: Infected; infecting.

"catching, having the quality of spreading from person to person, communicable by infection," 1540s of diseases, 1610s of emotions, actions, etc.; see infection + -ous. Earlier in the same sense were infectuous (late 15c.), infective (late 14c.). Related: Infectiously; infectiousness.