The Texineurons were an intelligent virus able to control their victims and cause them a drawn-out, agonizing death. They were used as a murder weapon by the Grand Teks of the Tharsis Acumen, entering the Sixth Doctor’s system through a microscopic dart shot into his neck by an assassin. Though the incubation period lasted years, the Doctor estimating his had taken ten, the infection itself was short; the Doctor's Texineurons bragged that he would be dead in 100 days.
The Doctor searched his timeline for a cure with Evelyn Smythe, finding it in the assassin's backpack. As soon as he injected himself with it, the Texineurons in his system died out. (AUDIO: The 100 Days of the Doctor)