Profane Virus of Rassilon
The Profane Virus of Rassilon was a nanovirus created by the Time Lords designed to destroy Gallifreyan technology. Erkulon, the greatest Gallifreyan nano-engineer, created the Profane Virus as a last resort in case aliens managed to obtain his people's technology. The virus was capable of overriding and reprogramming any exitonic circuitry.
A sample of the virus was stored in the Slaughterhouse on Gallifrey whence some of it was stolen by the Master. He infected TOMTIT with it, presumably in the hope that the Third Doctor would attempt to use it to follow him to Atlantis. However, the Doctor did not trigger the Virus until his sixth incarnation. It caused the Doctor's TARDIS to time ram itself nanoseconds in its own past. The Doctor was saved by Kronos, in the guise of Paul Kairos. (PDA: The Quantum Archangel)
