The Swarm (The Invisible Enemy)

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

Biology
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 Swarm could affect non-living sentients such as robots. (TV: The Invisible Enemy)

History
The Swarm lay dormant in cloud-like anomalies in space, including the area near Saturn for untold millennia, until spaceships passed through them. In the year 5000 it was picked up by the crew of a passing spacecraft. It infected the relief crew for the Titan Base, and through them eventually most of the Bi-Al Foundation, located on Asteroid K4067.

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 Mark I, Professor Marius, the crew of the Titan Base and most of the Bi-Al Foundation. (TV: The Invisible Enemy)

The Swarm was later encountered by the Seventh Doctor, when it took over the mind of Hector Thomas. It used Hector to commander the TARDIS to return to Titan Base. The Swarm ensured its own creation by returning to the Bi-Al Foundation, where it mutated from the Saturnian plague.

While in 4920, the Nucleus cloned itself and uploaded its mind to the Bi-Al Foundation computer. It then used the TARDIS to travel forward in time to the 52nd century, by which time the centre had become a Hypernet relay station. Using the TARDIS' dimensional stabiliser and the power of the Hypernet, it intended to recreate itself and become corporeal again. (AUDIO: Revenge of the Swarm)