The Kraal virus was a microorganism created by the Kraal scientist Styggron.
History[]
The virus 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 Kraals had to rely on their androids to transport the virus.
The only victim of the virus was Styggron himself, who was killed when an attack by the Fourth Doctor's reprogrammed android double led to a vial containing the virus being cracked open and the Kraal scientist being exposed to it. (TV: The Android Invasion [+]Terry Nation, Doctor Who season 13 (BBC1, 1975).)