The Flood (The Waters of Mars)

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

The Flood were a viral species of unknown origin, which existed 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. Other areas appeared to be slightly cracked as well, such as the forehead (visible on Andy Stone). 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. The infected can emit high-pitched screams, capable of cracking open ice. (DW: The Waters of Mars)

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)
 * 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.

Trivia

 * The Flood's real name is unknown, "the Flood" being a name the Doctor gives it.