The Flood is a sentient Martian virus, imprisoned thousands of years ago by the Ice Warriors. It was later released by the crew of Bowie Base One, humanity's first Martian colony. The Flood's release would subsequently cause the death of everyone in the colony, though three would be retroactivey saved by The Doctor. An infection of The Flood can by characterized by cracks on ones mouth and water dripping from it, a drastic change in one‘s voice and the ability to shoot water from one‘s hands. The infected commonly produce a loud, raspy growl. The virus can spread to anyone who makes direct contact with infected water and it can take anywhere from a few seconds to several hours to take over the host. The infected also effectively share a hive mind.
According to the Doctor, the Flood is a patient and intelligent pathogen that “has time as an ally”. It is a serious danger to any life-form in the galaxy, as just a single drop of infected water can contaminate and take control of a planet. However, it is vulnerable to electrocution, nuclear weapons and freezing temperatures. Even after the destruction of the base, however, there is a chance of its return, due to the amount of glaciers on Mars.