The Silurian plague (AUDIO: Hosts of the Wirrn) or Silurian virus was a deadly virus created by the Silurian scientist Bokka K'to (PROSE: The Scales of Injustice) when the Silurians ruled the Earth. They would use the virus 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 virus.
When the Silurians attempted to reclaim Earth, Morka had K'to infect Major Baker with the virus. Baker in turn infected Edward Masters, who later travelled to London, infecting several people at Marylebone station. The virus quickly escalated into epidemic across England, with it even reaching France.
The Third Doctor later created a cure for the virus 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 virus. (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)