The Flood (The Waters of Mars)

The Flood was a viral species which originated inside a glacier on the planet Mars. The Flood needed a physical form to take on, so they infected a number of crew members from Bowie Base One, starting with Andrew Stone. It went on further, with the part of The Flood in Andy infecting Margaret Cain and Tarak Ital. After infecting Ed Gold, Steffi Ehrlich, and Roman Groom, Maggie went to break the glacier, but failed due to Adelaide Brooke initiating Action Five.

Biology
The Flood's natural state was in liquid water. It was capable of controlling the water, causing it to flow in unnatural directions. It was also capable of infecting other life forms by coming into contact with them, making them spasm and convulse as they were taken over, mainly because it was a viral life-form.

Once infected, the individual constantly exuded water, pouring from the body; hands and mouth. This could be increased to produce high-pressure sprays from both the hands and the mouth. The infected victims 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. Upon mutation, infected victims could deactivate nearby cameras. When infected, the hosts would also develop cracked skin (especially around the mouth and neck) and blackened teeth, as well as an internal fission in the host's body that produced the water. Some hosts, though not all, could develop faded blue eyes. The Tenth Doctor suggested that this might mean they were closer to human than those with the faded blue eyes. 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. Hosts could also run extremely fast, even going as far as to keep up with a rocket-powered robot and 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 affected by electricity (which salty water conducts well); The infected Andy Stone was electrocuted by the Tenth Doctor, but was only momentarily incapacitated. However, the Flood creatures, being made almost entirely up of water and constantly producing it, could be destroyed by intensive heat. This weakness was shared with those most likely to have imprisoned them, 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 victims could understand Ancient North Martian. Victims could no longer speak English, but could emit gurgling growls and snarls. 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. Strangely, a victims' scream could be heard through the thickest of metals. (TV: The Waters of Mars)

History
At some point, the Flood was sealed inside a glacier in the Gusev Crater on Mars. The Tenth Doctor speculated it was by the Ice Warriors. In 2058 and 2059, the human colony Bowie Base One used the glacier as a water source.

Original timeline
On 21 November 2059, the Flood managed to infect Andy Stone when he ate a carrot he had washed thanks to a broken filter and the replacements not fitting, let them through into the water supply. The Flood proceeded to overrun Bowie Base One which eventually led to Captain Adelaide Brooke initiating Action Five. The resulting nuclear detonation destroyed Bowie Base One, the surviving crew, the infected personnel and the Flood.

Many on Earth mourned the crew's death, but remained unaware of why Adelaide had destroyed Bowie Base One or that it was to stop the Flood from reaching Earth. (TV: The Waters of Mars)

New timeline
In the altered timeline, the Tenth Doctor visited Bowie Base One at the same time that the Flood infected Andy Stone thanks to a broken filter. Reluctantly aided by the Doctor, the crew attempted to battle the Flood. The Flood learned of the abundance of water on Earth through the infected Margaret Cain and expressed a desire to reach Earth in order to spread. As the situation worsened, the Flood-infected Margaret, Andy, Steffi Ehrlich, Roman Groom and Tarak Ital started shattering the ice holding the rest of the Flood in order to release it.

Though the Doctor originally refused to help as the destruction of Bowie Base One was a fixed point in time, he declared himself the Time Lord Victorious and attempted to save Adelaide, Mia Bennett and Yuri Kerenski. With the help of Gadget, the Doctor managed to transport the survivors to safety in his TARDIS. Before departure, Adelaide initiated Action Five which activated a nuclear device under Bowie Base One. The resulting nuclear blast destroyed Bowie Base One, the infected crewmembers and the Flood.

Following Adelaide's suicide on Earth to correct the timeline, Mia and Yuri spread the story of the Flood infection of Bowie Base One and Adelaide's heroic actions to stop them. This inspired Susie Fontana Brooke to lead humanity into the stars. (TV: The Waters of Mars)