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 first humanoid race destroyed themselves and all life on their world with a genetically designed virus which eventually consumed itself. (PROSE: Falls the Shadow)

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. After its destruction, these viruses were scattered through time and space. The Daleks experimented on them in order to create a strain that converted lifeforms into Dalek mutants. (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)

A virus native to Mars known as The Flood, which apparently was sentient as a hive-mind, doomed Bowie Base One's inhabitants. (TV: The Waters of Mars)

Life itself is a virus that was sent back in time to the creation of the universe. (AUDIO: The Viyran Solution)