Virus

A virus was a biological disease-causing agent, although the term itself could also apply to a malicious technological computer program.

Biological
In the case of a biological virus, it could be transmitted through biological fluids (blood etc) or through basic contact.

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

Another virus originated from a mutation in a mood drug in the upper levels of New New York in New Earth, this one, however, was airbourne. (DW: Gridlock)

Technological
A technological virus functioned in a similar way as a biological virus, altering, changing or corrupting the target system.

For example, 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 destory the Earth and obtain nuclear fuel to sell on to other races. (DW: World War Three)