Virus

A virus is a transmittable disease through; technology, biology or psychic ability/telepathy.

Biological
In the case of a biological virus it can be transmitted through biological fluids (blood etc) or through basic contact. The werewolf that Rose Tyler and the Doctor encountered wanted to bite the Queen to pass on it's virus. (DW: Tooth and Claw)

The Faction Paradox also introduced a virus into the Third Doctor as he regenerated altering his biodata. (EDA: Interference: Book One)

Technological
A ttechnological virus functions in the same way as a biological virus, altering, changing or corrupting the target system. The Doctor gave Mickey Smith a virus to put out on the Internet to erase any signs of him following the Slitheen's attempt to obtain nuclear weapons. (DW: World War Three)