The Silurian plague (AUDIO: Hosts of the Wirrn) was a disease caused by a strain of deadly bacteria (TV: Doctor Who and the Silurians) or, by another account, a virus. It was created by the Silurian scientist Bokka K'to (PROSE: The Scales of Injustice) when the Silurians ruled the Earth. They would use the bacteria on the proto-human apes that raided their crop fields and killed millions of them. When the Silurians retreated into hibernation they kept samples of the bacteria.
When the Silurians attempted to reclaim Earth, Morka had K'to infect Major Baker with the bacteria. Baker in turn infected Edward Masters, who later travelled to London, infecting several people at Marylebone station. The outbreak quickly escalated into epidemic across England, with it even reaching France.
The Third Doctor later created a cure for the disease from a sample given to him by Okdel L'da. The cure was mass produced and sent out over England, making humanity immune to the bacteria. (TV: Doctor Who and the Silurians)
In an alternate universe manipulated into existence by the Monk, the plague successfully decimated humanity and allowed the Silurians to reclaim Earth. To the humans that survived it, the virus was known as "the Nightmare". (PROSE: Blood Heat)