The Flood (The Waters of Mars)

"Water is patient Adelaide, water just waits. It wears down the clifftops, the mountains, the whole of the World. Water always wins!"

The Flood was a viral species of unknown origin which existed 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 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. When infected, the hosts would also develop cracked skin (especially around the mouth), blackened teeth and faded blue eyes. These physical symptoms could be masked. The host also had the ability to survive on the surface of Mars, likely due to the fusion reaction which produced the water.

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. (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.