When the virulent Flood organism reaches a certain critical mass, it develops a centralized intelligence, which calls itself a Gravemind. This is the Flood personified, and it carries with it the genetic memory of millenia of slumber.

The thing was stopped once before by the Forerunner activation of the Halo weapon, but it will not be stopped this time by a weak alliance of ruined humanity or the deluded obsessions of the Prophet of Truth. The Gravemind intends to achieve its ultimate purpose - to consume and absorb every thinking creature in the Galaxy.

NOTE: A lot to chew on here. That any Flood unit can become a Gravemind under the right circumstances, also repeated in the information about general Flood units, seems to bolster the long-held theory that the Keyes Blob from Halo 1 was a proto-Gravemind. It is suggested here that each such individual contains the entire memory and experiences of the Flood as a species, which one presumes includes the memories absorbed from infected species, as we see Gravemind do so with the Prophet of Regret in Halo 2.

The text here seems to state unequivocally that the only thing that has stopped the Flood in the past is Halo activation, and that the remaining conventional forces of the Covenant and Humanity, however arrayed, are not enough to effect a victory by other means. Either a third possibility exists, or we may end up spending a good deal of time in Halo 3 trying to achieve what we spend the last two games trying to prevent: the activation of the Halo installations.