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 Cybermen created a neurotropic virus to infect 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)

The 456 ambassador released a virus into Thames House to kill everyone there and make a point. (TV: Children of Earth: Day Four)

In 2014, the Sentinels of the New Dawn intended to cause an ebola virus epidemic to take power. (AUDIO: The Sentinels of the New Dawn)

The Drellerans, who had suffered under the subjugation of Lord Jack Corrigan and his men, infected them with a virus which compelled them to return to Earth and spread the infection to the rest of humanity. After an encounter with the Fourth Doctor and Leela, they injected them with an anti-virus to cure the infected humans. (AUDIO: Destination: Nerva)

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 airborne. Incredibly fast and aggressive, the virus killed the entire surface population of New Earth in seven minutes flat, save for the Face of Boe and Novice Hame and those in the Undercity, which was sealed off in the nick of time. With no new victims to feed on, even the virus itself eventually died. (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 was a virus that was sent back in time to the creation of the universe. (AUDIO: The Viyran Solution)

A virus threatened the life of a dinosaur-like creature, along with a sentient cell culture (the Culture) hosted in it. The infection was eradicated by the Seventh Doctor with a dose of maxenshudicea. (COMIC: Culture Shock)