Virus

A virus was a biological disease-causing agent that could be transmitted through biological fluids (blood, etc.) or through basic contact.

As biological weapons
The mondasian Cybermen created a neurotropic virus (Neurotrope X) to affect and convert the inhabitants of the moonbase. (TV: The Moonbase)

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 Faction Paradox introduced a virus into the Third Doctor as he regenerated, altering his biodata. (PROSE: Interference - Book One)

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

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