The Flood (The Waters of Mars)

The Flood was a viral species which originated inside a glacier on the planet Mars. The Flood needed a physical form to take on, so they infected a number of crew members from Bowie Base One, starting with Andrew Stone. It went on further, with the part of The Flood in Andy infecting Margaret Cain and Tarak Ital. After infecting Ed Gold, Steffi Ehrlich, and Roman Groom, Maggie went to break the glacier, but failed due to Adelaide Brooke initiating Action Five.

Biology
The Flood's natural state was in liquid water. It was capable of controlling the water, causing it to flow in unnatural directions. It was also capable of infecting other life forms by coming into contact with them, making them spasm and convulse as they were taken over, mainly because it was a viral life-form.

Once infected, the individual constantly exuded water, pouring from the body; hands and mouth. This could be increased to produce high-pressure sprays from both the hands and the mouth. The infected victims would no longer draw their oxygen from air, but from water and they would be near walking corpses, as the infection greatly weakened their heartbeats and pulse. Also, the infection would cause a lot of electrical activity in the host's brain. Upon mutation, infected victims could deactivate nearby cameras. When infected, the hosts would also develop cracked skin (especially around the mouth and neck) and blackened teeth, as well as an internal fission in the host's body that produced the water. Some hosts, though not all, could develop faded blue eyes. These physical symptoms could be masked, meaning that the infection could hide inside a human host, and infect him or her at any moment.

The host also had the ability to survive on the surface of Mars. Hosts could also run extremely fast, even going as far as to keep up with a rocket-powered robot and outrun other running people. A single droplet of the viral water emitted by the host could pass on the infection to another host. Despite the considerable amount of water produced by the infection, the infected did not seem to be overly affected by electricity (which salty water conducts well); The infected Andy Stone was electrocuted by the Tenth Doctor, but was only momentarily incapacitated. However, the Flood creatures, being made almost entirely up of water and constantly producing it, could be destroyed by intensive heat. This weakness was shared with those most likely to have imprisoned them, the Ice Warriors.

The Flood proved capable of coordinated action despite being scattered, as well as considerable intelligence. It could not communicate verbally once infection manifested itself, although victims could understand Ancient North Martian. Victims could no longer speak English, but could emit gurgling growls and snarls. The infected could also emit terrifying high-pitched screams that sounded like tortured metal, capable of cracking open ice, and communicating with other infected victims. Strangely, a victims' scream could be heard through the thickest of metals. (TV: The Waters of Mars)

History
At some point, the Flood was sealed inside a glacier on Mars. The Tenth Doctor speculated it was by the Ice Warriors. In 2058 and 2059, the human colony Bowie Base One used the 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. (TV: The Waters of Mars)

Trivia

 * Before Series 10 was released, several filming pictures showed what appeared to be The Flood. Although it later turned out to be The Pilot.