Joystiq writes: "Shortly after learning that the Flood would be making an appearance in Halo Wars, we were whisked away to a meeting with Ensemble's Graeme Divine where we immediately asked: "Is the Flood a playable race?" As quickly as we asked it, we were answered: no. With an emphasis on the story-driven single-player campaign, Divine told us that the multiplayer component would only include the UNSC and Covenant forces, as already revealed, and the campaign would only be playable as the UNSC, not the Covenant.
But that brings up another issue: how is the Flood present in a prequel taking place 20 years before the events in Halo: Combat Evolved? The more Halo-aware amongst you (read: total nerds) will recall that the introduction of the mysterious Flood in that seminal title was a surprise to the player and, indeed, to the entire UNSC command. When asked if they're retconning the Flood earlier into the Halo timeline, Divine said that the events observed in Halo 1 "was the first time UNSC knew about the Flood." So how does the existence of the parasitic aliens twenty years earlier go entirely unrecorded? Unsurprisingly, they're not saying (our guess: everyone dies)."