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 were taken over, mainly because it was a viral life-form.

Once infected, the individual constantly produced water, gushing from the body, hands and mouth. This could be increased to produce high-pressure sprays from both the hands and the mouth. The infected victim 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, probably what gave the victim their great amount of intelligence. When infected, the hosts would also develop cracked skin (especially around the mouth and neck), 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, 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, likely due to the fission reaction which produced the water and also because they breathed water instead of air. Hosts could also run extremely fast, even going as far as to keep up with a rocket - powered robot and being able to 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 effected by electricity (of which water conducts extremely well); 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 stun a flood victim, but not kill him or her. However, the Flood creatures, being made almost entirely up of water, and constantly producing it, could be destroyed by intensive heat, a weakness that they shared with their 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, although a victim could understand Ancient North Martian. 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. (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 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. (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.