The Flood are coming back in Halo 4. Don't rant and rage over the return of the series' least-loved enemy yet, however. It seems that the alien zombie parasite things will only appear in a single multiplayer game mode, appropriately called "Flood" (via CVG).
Flood compares to the Infected mode of previous games, with two "infected" Flood players taking on up to eight others. Each kill scored by an infected adds the downed player to the Flood's ranks until there's no one left to kill. 343i revealed at San Diego Comic-Con 2012 that the Flood would not be appearing in Halo 4, news that was cheered by the gathered crowd. It looks like we can amend that promise to "the Flood won't appear in Halo 4's story."
A newly released selection of screens (via All Games Beta) offer a first look at Halo 4's Flood, as well as two new vehicles: a troop carrier and a rocket-firing mech.