An infestation was the large presence of or invasion by pests, parasites, insects, animals, or other organisms in a place, to such an extent that it could cause damage or disease. Infestation could also refer to organisms living on or within a host.
In 2267, the Earth probe Nomad's function was to probe for biological infestations and destroy that which was not perfect. According to Captain James T. Kirk, there was never any probe sent out for biological infestation. Nomad destroyed the Malurian race, a population of four billion on four planets, because it considered them an unstable biological infestation. With Nomad's intention of returning to Earth, Spock noted that it would find the planet infested with imperfect buological units. Kirk replied that Nomad would then carry out its prime directive to sterilize. (TOS: "The Changeling")