The Flood is a form of an infection they can infect almost any thing! They fought with the Forerunners in the Flood war and are the reason the Forerunners are extinct. The Flood can be found wandering around Halos rings and in a Covenant super carrier they hijacked. In there Mega Bloks set there are 3 types of flood starting with a infected elite , infected marine , and 3 infectors. The set will be released in late August 2011 or in Fall 2011. The set is inspired by a campaign level in Halo 3 called "FloodGate" and they first appeared in a Halo 1 level called "GuiltySpark". The Flood have a leader called the Gravemind but I doubt he'll be in any sets for a while.