infestation (n.)

late 15c., "to attack, assail, hurt, distress, annoy," from Old French infester (14c.), from Latin infestare "to attack, disturb, trouble," from infestus "unsafe, hostile, threatening, dangerous," originally "inexorable, not able to be handled," from in- "not, opposite of" (see in- (1)) + -festus, perhaps "(able to be) seized" (see manifest (adj.)). Sense of "swarm over in large numbers, attack parasitically" first recorded c. 1600. Related: Infested; infesting.