The Flood (The Waters of Mars)

The Flood were a viral species of unknown origin, which exsisted on the planet Mars.

Biology
The only thing known about the Flood's natural state is that it exists in water and can infect other lifeforms. One drop is enough to infect someone.

Once a lifeform has been infected, it constantly produces water; the liquid dribbles out of the host's mouth and soaks through their clothes. It can produce further water to break through barriers and locks, and blast high-pressure sprays from their mouths and arms. The infected hosts show parched, cracked mouths, blackened teeth, and, after a while, faded blue eyes. Infection takes place quickly, but the Flood can cover up the physical symptoms for a short period of time. Amazingly, the infected hosts were capable of surviving on the surface of Mars without any form of lifesupport assistance. This was believed to be attributed to the fission reactions caused by the viral organisms and was seen with the cracked lips around the hosts mouth.

It proved capable of coordinated action despite being scattered, indicating it can communicate telepathically. It cannot communicate verbally once infection has manifested itself(however it can communicate verbaly,and was shown to in English, while the infection was manifesting itself). The infected can emit high-pitched screams, capable of cracking open ice.

History
At some point, the Flood might have been sealed in a glacier by the Ice Warriors. They recognise the Ancient North Martian language when the Doctor speaks it. In 2058 and 2059, the human colony Bowie Base One used that glacier as a water source. Due to a broken filter system, the Flood was able to infect the colony's biodome workers in November 2059. After becoming aware of Earth and its abundance of water, it attempted to infect the entire colony and reach Earth; the colony was destroyed to prevent them escaping Mars. (DW: The Waters of Mars)

Name

 * The Flood's real name is unknown, "the Flood" being a name the Doctor gives it.
 * The Flood from Doctor Who bears similarities to the Flood from Halo; both are parasitic creatures which control hosts, terraform environments, possess a form of telepathic communication and were sealed away by a more powerful race (abeit for different reasons) for fear of what they might do. Additionally, both species show an indifference to the host's condition: the Doctor Who Flood warps the skin and facial features and use the body as a means of water production, while the Halo Flood completely transforms the host and uses it for food. Furthermore, in both cases the organism completely controls the host but has access to its memories. The major difference is that the Doctor Who Flood can infect via a single drop of water while the Halo Flood have more complex infection forms. For these reasons the Flood in Doctor Who may have been named after the Halo Flood.