Infestation refers to the state of being invaded or overrun by pests or parasites. It can also refer to the actual organisms living on or within a host or The condition of being invaded or inhabited by ectoparasites such as lice, mites or ticks.External or ectoparasitic infestation is a condition in which organisms live primarily on the surface of the host and includes those involving mites, ticks, head lice and bed bugs.An internal infestation is a condition in which organisms live within the host and includes those involving worms.