Were you looking for the Flood, a zombie-like species inhabiting the Halos or Infection, a multiplayer gametype staring in Halo 3?
Zombies is a gamemode in Halo 2. It is unique among Halo 2's gametype selection in that the mode originated as an honour rules-based gametype as a variation of Slayer, before being released by Bungie as a game setting as a rare piece of downloadable content. The mode was eventually brought back in Halo 3 and all subsequent games as Infection (or the similar Flood mode).
The basic rules of Zombies 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 will switch to the green team. The ultimate goal is to last as long as possible without getting killed by the Zombies, and becoming one themselves.
The mode originated in Halo 2 custom games as a variant of the Slayer gametype. This mode was based on honour rules, meaning players were trusted to manually switch teams when they were killed. However in June 2005, the mode was released as an official ruleset for Halo 2 in Issue 45 of the Official Xbox Magazine. The mode was included in a demo disc bundled with the magazine, and had the installation for the mode included. The gamemode could be installed to the Xbox's hard drive and then played in Halo 2 as a mode like any other.