Ood

The Ood were a gestalt race of humanoids.

Biology
The Ood were a humanoid species with coleoid tentacles on the lower portions of their faces. They have no vocal cord and required translation spheres in order to communicate with other species. In order to communicate with each other, the Ood shared amongst themselves a low-level telepathic communication field, rated at "Basic 5". The colour of an Ood's eye change with the level of telepathic activity, glowing bright-red when there is a strong telepathic source. There are no male or female Ood, which would suggest that they are hermaphrodites. (Doctor Who: The Visual Dictionary)

Humans misinterpreted the presence of red eyes as a disease, known as "red-eye".

Originally, the Ood were a hive mind led by the Ood Brain. This kept control of the two brains in each Ood (the second of which was what was removed and merged with the translation sphere by their Human masters. The removal of this brain and the containment of the Ood Brain made them willing to serve and incapable of surviving on their own. (DW: Planet of the Ood)

History
The Ood were a peaceful race, evolving in the cold climate of the Ood Sphere. In the 3800s, Humanity discovered the Ood, enslaved them and used them to perform menial tasks throughout the three galaxies of the Second Great and Bountiful Human Empire. The changes made to the Ood's physiology by Ood Organisations meant that they were willing for this to happen to them. (DW: Planet of the Ood)

By the year 43K2.1, it was claimed that every Human had an Ood servant. They were generally barely even regarded as being alive, wend are considered to be expendable. Despite this the "Friends of the Ood" were lobbying for Ood freedom. (DW: The Impossible Planet)

When encountered by the Doctor and Rose Tyler, a group of Ood were part of the Walker Expedition. Unlike the Humans or the Doctor, the Ood knew of prophecies regarding the Beast which the expedition would later discover. (TS: Tardisode 8, Tardisode 9, DW: The Impossible Planet)

The empathic nature of the Ood seemed to make them susceptible to psychic possession by the Beast, who formed the Ood on the base into its "Legion". (DW: The Impossible Planet)

The Ood were defeated when Danny Bartok, the expedition member in charge of them, broadcast a telepathic flare which reduced their field to "Basic Zero", creating a "brainstorm" which caused them to collapse. However, their telepathic field began to reassert itself after a time. When Krop Tor was sucked into the black hole, the Doctor was unable to save any of the Ood on the base, who had been freed of the Beast's control, and all of them perished. All of them were given posthumous honours. (DW: The Satan Pit).

In 4125, the Doctor and Donna Noble visited the Ood Sphere and Ood Organisations, and decided to find out about the origins of the Ood and what turned them into a slave race. At the same time, the "red-eye" disease was becoming more and more prominent. After discovering that the disease was caused by the Ood Brain's subconscious reasserting itself over the Ood (thanks to the efforts of one member of the Friends of the Ood who had infiltrated the company), the Ood were set free and allowed to roam their homeworld in peace. (DW: Planet of the Ood)