A space plague was a type of disease.
The Daleks caused a space plague with "plague missiles", striking Terran colonies. The only cure was parrinium which was sought by both humans and Daleks on the planet Exxilon in the Exxilon Gambit. Though the Daleks claimed to need it to cure their own afflicted colonies, (TV: Death to the Daleks) historians who later studied this era were divided on the truthfulness of this. (PROSE: The Whoniverse) Dalek-focused historians insisted the exterminators, having created the plague, were indeed unaffected. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) One account even had the Daleks themselves admitting that they were immune to the space plague. (PROSE: Death to the Daleks) The Time Lords also understood that the plague, though having been initially assumed to be natural in origin, was in fact a chemical weapon attack by the Daleks designed to weaken the ever-expanding Terran empire. (PROSE: Dalek Combat Training Manual) A dissenting account claimed that the plague was merely an opportunity that the Daleks had exploited, and that they were immune to it as a result of their work in curing the Movellan virus which had earlier afflicted them. (PROSE: The History of the Daleks)
A new strain of the space plague, also effectively cured by parrinium, later also affected the Daleks themselves. The Daleks tried to invade Exxilon once again to secure all the parrinium for themselves, while the humans first brokered a deal with the Exxilons to obtain some. (AUDIO: The Dalek Protocol)
During the mid-21st century, the Second Doctor (PROSE: The Dark Path) helped the Draconians deal with an outbreak of space plague. He was ennobled for his efforts. (TV: Frontier in Space) This outbreak was specifically identified as part of the Great Space Plague. (PROSE: Catastrophea)
A space station threatened by a space plague was saved by disinfectodroids. However, their obsessive programming made them clean up everything, including the station occupants, and dumped them all on an alien planet. The Tenth Doctor and Rose Tyler were sent to the same planet when they arrived on the station. (COMIC: The Germ War)
Alternate timeline[]
In one alternate timeline in which the UK-201 never crashed on Dido and made it to the planet Astra, Vicki Pallister's daughter, Carla Dalton, left to aid the Outer Provinces during an outbreak of space plague. She died in a rebel attack. Some years later, Jeran Dalton, Vicki's husband, attended a conference seeking to deal with the spread of the plague. While there, he succumbed to the disease himself. Vicki later altered the timeline further, saving both their lives. (AUDIO: The Crash of the UK-201)