Just one drop. That's all it takes for the Flood to infect a host. It's hidden within the water on Mars - so dangerous that classic creatures the Ice Warriors encased the virus within ice. In fact, it's so dangerous that Adelaide had to go all nuclear to stop them which provoked the 'Time Lord Victorious' from the Tenth Doctor, pushing him into a position where he had to break the laws of time itself to get away. Part of what makes the Flood so effective is that it seems almost primal. It just finds more and more hosts, turning them into grotesque water zombies with cracked lips and pale eyes. It doesn't care who it affects, taking both old and young, leading to heartbreaking scenes as it claims the crew of Bowie Base One. However, it's also sentient, expressing a wish to travel to Earth due to the vast reserves of water there - so stopping them quickly becomes a matter of saving the planet. Of course, a number of creatures have tried to take Earth and have been stopped, but as very few almost trap the Doctor in an unwinnable situation (that he only escapes by travelling away in the TARDIS), the Flood manage to be one of the most threating and effective new series aliens.
