Computer virus

Computer viruses were software programs used to infect computers for various reasons, usually malicious. The Ninth Doctor gave Mickey Smith a computer virus that would erase all evidence of his existence from the Internet. (TV: World War Three)

A Bad Wolf virus at some point erased all information on Rose Tyler from the Torchwood database. (TV: 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. (TV: The Lost Boy)

The Eleventh 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." (TV: The Eleventh Hour)