Infection

Looking for the Flood Infection Form?  Infection' is a multiplayer gametype in Halo 3 and Halo: Reach''. It is the official Zombie mode, modeled after zombie mode's popularity as a custom game in Halo 2.

Description
The object of the gametype is for the players to stay "uninfected" as long as possible while the zombies try to kill every living player. The number of players starting out as Zombies, "Alpha Zombies", can be changed in the options and are selected at random from the start. When a Zombie kills a "living" player, they are "infected" and they become a zombie. When there is just one living player left, he/she is declared "Last Man Standing".

Medals

 * Zombie Killing Spree - Kill five zombies without dying.
 * Hell's Janitor - Kill ten zombies without dying.
 * Infection Spree - Infect five humans without dying.
 * Mmmm Brains - Infect ten humans without dying.
 * Last Man Standing - Be the last human left alive in the round.
 * Is Quisnam Protero Damno - Kill fifteen humans without dying. (removed)

Halloween
On October 31, 2007, a specialized Halloween Matchmaking playlist was available, which played this game mode with up to 13 players (it being an "unlucky" number). This game type's popularity allowed it to become a "regular" among Double EXP Weekend games. It was repeated again on October 31 2008, but unlike other occasions, the October 31 2007, 2008, and 2009 ones were Ranked. The others were mainly Social.

Double EXP Weekend
On April 10th, 2008, the playlist came back for a Double EXP Weekend as Living Dead, also starting with exactly 13 players. Living Dead also appeared on the weekend of May 22 and on the 28th August. Living Dead is a Double EXP Weekend game with many games. Infection games have also appeared in other Double EXP playlists such as Turkey Day, the 2008 Thanksgiving weekend playlist.

Tactics and Tips

 * If you are making an Infection game, try to make it very balanced. For example, give the zombies 200% health but no shields and only slightly higher speed and jump. In turn, give humans Shotguns and Pistols with limited ammo. Also, when making Forge maps, if you make a hiding room for survivors don't make it impossible for zombies to get in. Instead, give the room two entrances the survivors must work together to guard. Another fun thing to do is to put breakable or movable items such as barrels and palettes in front of doors to barricade them.
 * By default, regular zombies use Energy Swords, hence has limited/short attack range, and cannot pick up weapons. As such, it is recommended for the player to be equipped with a ranged weapon such as the Battle Rifle or the Covenant Carbine.
 * If you are playing as a zombie, its a good idea to put on Active Camo, so you can sneak up unnoticed or stalk the humans from behind, making sure they don't see you.
 * Yet another tactic that the "living" use is called "baiting". The players gather in a small, confined space; preferably one with only one opening. Whenever the zombies' attacks lull, the players send out a freelancer, known simply as "bait". The "bait" will attempt to lure the zombies closer to the stronghold, where the trap is set off and the zombie is killed by overwhelming force.
 * Sometimes it is a good idea to commit suicide if your group is under assault by a large zombie hoard. Especially if there are only 2 of you left. Try not using Spike, Plasma, or Firebomb Grenades to kill yourself, because the explosion could damage your teammates.
 * Try to not use Gravity Hammers or Energy Swords when possible, in most types the zombies have one or both of these weapons and such close range combat would result in you being killed often.
 * A team that regroups in a single area and holds their position in a well defensible location has a much higher chance of surviving.
 * Conserving ammunition is key in all Infection game types, therefore weapons that either fire single shot or bursts such as a Battle Rifle or Carbine are vital if a team is to survive.
 * Accuracy and shot placement are equally important; most players tend to aim for the one spot where it will do the most damage, the head. However, from long range, targeting this part of the body can be tricky, even though the head is less shielded and armored when compared to the rest of the body, it is also a small target. Shooting at the center of mass, the torso or legs, guarantee's better chances of success. In addition, jumping or crouching makes shot placement on the head difficult; the main body isn't as easy to conceal or dodge with.

Trivia

 * "Infection" is based on the popular custom gametype in Halo 2 known as "Zombies". Zombies was based on Team Slayer, with the Red team being Humans and the Green team being Zombies. The Humans could only use human weapons, and the Zombies could only use Energy Swords. The game began with just one or two Zombie players, who would attempt to hunt down and kill the Humans. When a Human was killed by a Zombie, they would have to change teams and become a Zombie. Since there were no facilities in Halo 2 Custom Games to limit what weapons each team could use or to force players to switch teams upon death, Zombie gametypes relied on Honor Rules.
 * New, but simple to obtain achievements are made just for Infection on the Mythic Map Pack, they are Delicious Brains, and Zombie Repeller. Delicious Brains is awarded when the player infects two humans, and Zombie Repeller is awarded by getting two zombie kills as a human.