Virus

A virus was a biological disease-causing agent that 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. (PROSE: Interference - Book One)

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

The race known as the Viyrans were known for cataloguing and creating various biological weapons such as viruses, some of which were stored in the Amethyst Viral Containment Station. (AUDIO: Patient Zero)

The Daleks at one time experimented on viruses in order to create a strain that converted lifeforms into Dalek mutants. (AUDIO: Patient Zero)

The Free Time organisation created a virus deadly to Time Lords, known as the Dogma Virus. (AUDIO: Gallifrey)