5000

5000 was, according to the Fourth Doctor, the year of the Great Breakout, a time when humanity surged across the galaxy like a tidal wave. A ship en route to Titan Base encountered a virus of unknown origin. The Doctor identified it as a sentient virus known as the Swarm. After the Doctor was infected by the Swarm, Leela and he went to the Bi-Al Foundation, where they were helped by Professor Marius and K9 Mark I to defeat/cure the virus. When they returned to the TARDIS to depart, the professor gave them K9. This began the Doctor's long association with the K9 series of robots. (DW: The Invisible Enemy)