5000


 * A ship en route to Titan Base encounters a virus of unknown origin, it is revealed through the intervention of the Doctor to be a sentient virus known as The Swarm. Through his intervention the Doctor is infected and transported by Leela of the Sevateem to the Centre for Alien Biomorphology, where both she and the Doctor are assisted by Professor Marius to defeat/cure the virus. (DW: The Invisible Enemy)