The Swarm (The Invisible Enemy)

The Swarm was a parasitic, virus-like entity which could assume many forms.

History
The Swarm hung dormant in a cloud-like area of space near Saturn for millennia, until in 5000 it was picked up by the crew of a Titan shuttle. It infected the relief crew for the Titan Base, and through them eventually most of the Bi-Al Foundation, also located on Titan.

The Swarm worked like a disease, spreading through electrical pulses which needed to make contact with a victim's eyes. This would turn victims into agents of the Swarm, answering to a nucleus, a microscopic and virus-like sentient organism that controlled the swarm. Visually, an infected lifeform could be recognised by the growing of grey filaments around their eyes and hands. There was no physical matter for the virus, which existed in the mind-brain interface. The Nucleus itself could speak, and had a crustacean-like form inside the Doctor's body. It retained this form later, when it had enlarged in the macrocosmic world. The Swarm could affect non-living sentients such as robots, but would have a diminished effect on individuals that relied heavily on instinct rather than reason.

Certain individuals (such as Leela) had antibodies that could defeat the virus. Additionally, the progress of the virus could be slowed simply by not thinking. Known victims of the Swarm included the Fourth Doctor, K9, Professor Marius, the crew of the Titan Base and most of the Bi-Al Foundation. (DW: The Invisible Enemy)