The Swarm (The Invisible Enemy)

The Swarm is a sapient microscopic viral species which invaded the Sol System in the year 5000 AD, after staying dormant in space for millennia.

Biology
The Swarm is a superorganism controlled by a microscopic crustacean-like creature known as the Nucleus of the Swarm. It has been described as both a virus and a noetic life form, which inhabits the mind-brain interface of its host. In this form, the Nucleus is a purely mental creature and has no ascertainable mass or structure. Due to its noetic nature, the Swarm can equally infect organic, biomechanical and robotic life forms, as long as they are intelligent, and be transmitted from one host to another by eye contact.

Outside of a host, the Nucleus can survive (presumably in a corporeal state) and spawn numerous eggs to create a new Swarm. They can remain dormant in space for millennia waiting for a new host.

Humanoids who have been infected by the Swarm are mentally controlled by it, and develop physical traits such as silver scales on their faces and hair-like silver filaments on their hands and around their eyes. Some Humans are naturally immune to the Swarm, and produce antibodies that can be used to cure the infected.

Appearances

 * Doctor Who, season 15, The Invisible Enemy (1977)