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 on New Earth; this one, however, was airbourne. (DW: Gridlock)

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

The Daleks, at one time experimented on viruses in order to create a strain that converts lifeforms into Daleks. (BFA: Patient Zero)

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

Technological

 * See computer virus