The Flood (The Waters of Mars)

The Flood was a viral species which orginated inside a glacier on the planet Mars.

Biology
The Flood's natural state was in liquid water. It was capable of controlling the water, allowing it to flow in unnatural directions. It was also capable of infecting other lifeforms by coming into contact with them, causing them to spasm and convulse as they are taken over.

Once infected, the individual constantly produced water, dripping from the body, hands and mouth. This could be increased to produce high-pressure sprays from both the hands and the mouth. When infected, the hosts would also develop cracked skin (especially around the mouth), blackened teeth and faded blue eyes, as well as an internal fission in the host's body that produced the water. These physical symptoms could be masked. The host also had the ability to survive on the surface of Mars, likely due to the fission reaction which produced the water. A single droplet of the 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 effected by electricity (of which water is a conductor); The infected Andy Stone was electrocuted by the Doctor, but was only momentarily incapacitated, but it was still a defence that could be used to damage a flood victim, but not kill him or her. However, the Flood creatures, being made almost entirely up of water, could be destroyed by intensive heat, a weakness that it shared with its imprisoners, 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. The infected could also emit high-pitched screams, capable of cracking open ice and communicating with other infected victims. (DW: The Waters of Mars)

History
At some point, the Flood was sealed inside a glacier on Mars, likely by the Ice Warriors.

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)


 * It is also quite likely that the virus itself was destroyed in the base's explosion, as Bowie Base One was situated almost directly on top of the glacier holding the Flood.