Computer virus

Computer viruses are software programs infecting computers for various, usually malicious reasons. The Doctor gave Mickey Smith a computer virus that would erase all evidence of his existence from the Internet. (DW: World War Three). Mickey considered using the virus as instructed but opted not to, explaining the matter in a post to his website. (WEB: Who Is Doctor Who?)

A Bad Wolf virus had at some point erased all information on Rose Tyler from the Torchwood database. (DW: Love & Monsters)

Cyberterrorists developed the Armageddon virus to bring down the international banking network, and the virus was later used by Alan Jackson to reprogram Mr Smith. (SJA: The Lost Boy)

The Doctor wrote a computer virus on a mobile phone. It caused all computers and networks to reset their counters and display only the number zero, to alert the Atraxi of Prisoner Zero's presence. It was also, according to the Doctor, "a tiny bit alive." (DW: The Eleventh Hour)