The Profane Virus of Rassilon was a nanovirus created by the Time Lords. It was designed to destroy Gallifreyan technology. Erkulon, the greatest Gallifreyan nano-engineer, created the Profane Virus as a last resort if aliens obtained his people's technology. The virus could override and reprogramme 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 hoping the Third Doctor would try to use it to follow him to Atlantis. However, the Doctor did not trigger the Virus until his sixth incarnation. It made the Doctor's TARDIS time ram itself nanoseconds in its own past. The Doctor was saved by Kronos, in the guise of Paul Kairos, who diverted enough of the nanovirus into the higher dimensions for the TARDIS to transmit a warning to its past self. (PROSE: The Quantum Archangel)