The Skreeth were a species that appeared on several planets at once, infecting the inhabitants by transforming them into beasts. The Time Lords sent the Doctor to investigate. He reported back to the Time Lords, and he also informed the Draconians about the Skreeth. The Draconians, in response, wiped out hundreds of planets where they suspected the Skreeth had appeared. This resulted in the Skreeth being pushed to the farthest ends of the galaxy.
The Seventh Doctor and Ace encountered the Skreeth on Earth in the 17th century. They had infected Matilda Brewer. The Doctor removed the infection by keeping her in the TARDIS for the rest of her life. (PROSE: The Devil Like a Bear)