The Ablative

The Ablative was a bacteria which reduced multi-cellular life to a comatose state.

It was created by the First Doctor in an experiment which he conducted while at the Time Lord Academy, with the ensuing scandal nearly getting him expelled until it was covered up by the Academy, who believed that all of the samples had been destroyed. The Rani later recalled this incident to the Sixth Doctor. (AUDIO: Planet of the Rani)