"The Ice Warriors...perhaps they found something down there..used their might and wisdom to freeze it" - The Doctor
The Flood was a viral species which originated inside a glacier on the planet Mars. It was a very dangerous infection which needed a suitable host.
The Flood was so deadly that the Ice Warriors, the native species on Mars sealed it inside a glacier.
Humans which are infected by the flood, lose their ability to speak proper English and only make grunts and screams although they can understand ancient North Martian. Their mouth becomes cracked and water spills out. They can also produce water and use it to infect others.
When the Flood learned of Earth's abundance of water they tryed to escape to Earth and infect the entire human race.
In 2059 a base was built on Mars called Bowie Base One, it was built on top of the glacier so they could retrieve the water. 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 Captain Adelaide Brooke initiating Action Five which destroyed the base.
The human colony Bowie Base One used the 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, although the Doctor who arrived earlier saved some of the crew.