Silurian bacteria. It's repeatedly called a bacterium in the story.
Talk about it here.
When the Silurians attempted to reclaim Earth, Morka released the virus and caused an epidemic across England. The Third Doctor created 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. (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)