The Flood is a sentient virus imprisoned in a glacier on Mars 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 and spread would later cause the colony's destruction killing everyone in it (however, three survived thanks to The Doctor). An infection of The Flood is characterized by cracks on a host's upper lip mouth with water dripping from it, a major change in the voice of a host, and the ability to shoot water from their hands. A common vocal noise for an infected individual is a loud, raspy growl, which a voice analyzer can still distinguish who he/she is with ease. The ability to shoot water from their hands is one of two methods to spread the virus. The second way the pathogen can be spread is by having an infected being jam its thumb against his/her head hard enough to make a wound and blast water into it. The time it takes for the organism to take over the host varies depending on its location and the presence of infection candidates, ranging from seconds to as long as several hours. The Flood can also be a hive mind among those who are infected.
According to the Doctor, the Flood is a patient and intelligent pathogen that has time as an ally. This thing is an extreme danger to any life-form in the galaxy because it takes one drop in a world's water supply for it to take control of a planet. However, like other threats in the galaxy it has many weaknesses and flaws. Electricity is a good weapon because it and water are a bad combination. Nuclear explosions are also good since it was demonstrated by the Martian colony. Freezing temperatures are also injuring against it since ice imprisons them. After the destruction of the base however there is a chance of it returning due to the amount of glaciers on Mars. It is possible that it might be on other worlds.