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 on Titan.

The Swarm worked like a disease, spreading by 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 their growing of grey filaments around their eyes and hands. There was no physical matter for the virus, which existed in a state in the mind-brain interface. The Nucleus, which could speak, had a crustacean-like form inside the Doctor's body, and later, when it had enlarged, in the macrocosmic world. The Swarm could therefore affect non-living sentients such as robots and would have a diminished effect on individuals that relied on instinct rather than reason.

Certain individuals (such as Leela) had certain antibodies that could defeat the virus. Additionally, 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)