Zombies

 Were you looking for The Flood, a zombie-like species inhabiting the Halos or Infection, a multiplayer gametype in Halo 3 and Halo: Reach''? ''

Zombie (or Zombies, Pandemic, and Dawn of The Dead) is a player-created game variant originating in Halo 2, although variants are also played in Halo PC. It's roots are now untraceable, and has become so popular that it has been translated into Halo 3 as Infection, with few minor edits and many more custom game tools. The basic rules involve two teams: a small starting green (or blue in some cases) team and a relatively large red team. There are no shields; red sports shotguns and magnums and green (the zombies) sports energy swords. Whenever a red team member is killed, they must switch to green team. The ultimate goal is to last as long as possible without getting killed by the Zombies, and becoming one themselves.

Halo 3 and Halo: Reach
The Gametype Infection features the living death known as Zombies. With quotes such as Infected and New Zombie commonly appearing. The aim is for the Zombies to kill all of the living to join their ranks. Although people can do the Zombie gametype from Halo 2 in Halo Slayer, Zombies have now been given their own, specific game.

Trivia

 * Initially, the game variant has honor rules, meaning there is no in-game mechanic that forces the a red player to switch teams after being killed (infected). Halo 3's Infection Mode, however, forces humans to become zombies after being killed, eliminating the honor rules.