The Flood (The Waters of Mars)

"The Flood" are a viral species of unknown origin, existing on Mars.

Biology
The only thing known about the Flood's natural state is that they exist 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. They 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. Infection takes place quickly, but the Flood can cover up the physical symptoms for a short period of time.

They proved capable of coordinated action despite being scattered, indicating they can communicate telepathically. They can communicate verbally, but rarely do.

The infected can emit high-pitched screams, capable of cracking open ice.

History
At some point, the Flood were sealed in a glacier by the Ice Warriors. They still recognise Ancient North Martian language.

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 were able to infect the colony's biodome workers in December 2059. After becoming aware of Earth and its abundance of water, they attempted to infect the entire colony and reach Earth; the colony was destroyed to prevent them escaping Mars. (DW: The Waters of Mars)

Trivia

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