The Kraal Virus was a virus created by the Kraal scientist Styggron. It was designed to kill off the human species in three weeks, and then disappear, allowing the Kraal to conquer the planet without opposition.
The virus could be transmitted by touch or by ingestion, but not by air. This meant that the Kraal had to rely on their androids to transport the virus. Styggron was killed when he was exposed to the virus. (TV: The Android Invasion)