What is the X in X-mas?
late 15c., "to attack, assail, hurt, distress, annoy," from Middle French infester, from Latin infestare "to attack, disturb, trouble," from infestus "hostile, dangerous," originally "inexorable, not able to be handled," from in- "not, opposite of" (see in- (1)) + -festus "(able to be) seized." Sense of "swarm over in large numbers" first recorded c.1600. Related: Infested; infesting.
infest in·fest (ĭn-fěst')
v. in·fest·ed, in·fest·ing, in·fests
To live as a parasite in or on tissues or organs or on the skin and its appendages.
To inhabit or overrun in numbers large enough to be harmful, threatening, or obnoxious.