Antoim

The Antoim were aliens described by Vicki Pallister as "great round things covered in spikes that float and smother and kill." The Fifth Doctor noted that they were remarkably stupid.

Vicki and Steven Taylor were being chased by them when they met the "Locum" Fifth Doctor in the TARDIS before a trip to Ravenna in the year 540, the First Doctor having just assisted a human colony in defeating them.

At some point after this encounter, the Doctor realised that the Antoim were an intelligent virus who passed through a dimensional instability making them expand thousands of times larger than they should be. When the Fifth Doctor- currently erased from history through the actions of the Monk- used the Antoim to blackmail the Monk into restoring his place in history, the Doctor was then able to use parts from the Monk's TARDIS to create a device that shrank them back to their normal size. The Ostadi then released an antibody that dealt with them. (AUDIO: The Secret History)