Silurian plague

The Silurian virus was a deadly virus used by the Silurians millions of years ago when they ruled the Earth. They would use the virus on the proto-human apes that raided their crop fields and it killed millions of them.

When they attempted to reclaim Earth, Morka released the virus and caused an epidemic across England. The Third Doctor was able to create a cure for the virus from a sample to him by Okdel. The cure was mass produced and sent out over England, making humanity immune to the virus. (DW: Doctor Who and the Silurians)