Time fungus

The time fungus was a parasitic fungus that could grow itself in different time zones to protect itself. In July 1900, the fungus inhabited the attic of 10 Downing Street and fed on the structure of the building itself, as well as several of the inhabitants.

Some of the house servants convinced themselves that they could "feed" certain residents to the fungus by helping it absorb certain traits from key people, in the hope that the fungus would eventually evolve to a level where it could become a superior prime minister. The Doctor dismissed this idea as nonsense as the fungus was incapable of sentience.

When the servants tried to arrange for the funus to infest the TARDIS so that it could be spread across all of time and space, the Doctor tricked them into thinking that the fungus had infected the ship already and then released another type of fungus into the building before he departed. This new strain of fungus was completely benign but spread very rapidly, the Doctor confident that the time fungus would be unable to defeat it without weakening itself to a level where conventional methods could destroy it. (AUDIO: Upstairs)