Infection originated as a custom gametype in Halo 2. In all Halo 3 variants, a set number of players will spawn as "Zombies". These players have modified characteristics; most often, said players will have increased movement speed, lowered gravity, lowered damage resistance, and will be equipped with an Energy Sword. When an infected player kills a healthy player (referred to as "humans"), that player respawns as a zombie. Outside of the Living Dead [Double EXP Weekend], this gametype appears only in the Rumble Pit playlist.
There are many variants of Infection. The most common are as follows.
Zombies are fast, but weak.
 Save on Bullet
Zombies are slightly faster than humans and have shields.
 Creeping Death
Zombies are invisible and equipped with swords and gravity hammers.
 Creeping Rockets
Zombies are invisible and equipped with Swords and Gravity Hammers. Humans spawn with Rockets.
 Speed Demons
Zombies are extremely fast, but weak. Humans spawn with Battle Rifles.
 Halo 2
Although Halo 2 did not have an actual Infection gametype, it was still a very common game played in custom games, often named simply "Zombies". Since there wasn't a gametype that could forcefully switch people to other teams upon death, players had to do so manually. This would often lead to people cheating by simply staying on the human team, rather than switching to the zombie team (which was almost always the green team) upon death.
The most common map that this was played on was Foundation, as the human team had significant protection within the small rooms around the outer ring of the map. Players would often jump on top of the back ledges within the rooms, where the zombie(s) would have a difficult time reaching them.
Infection games are mostly about personal preservation, with a little bit of teamwork involved. At the beginning of a game it is advised that players stick together in a large group covering all of the access points to the group of players. This should keep the opposing Infected players away for quite some time. During this first phase of the game there is no real need to get a huge amount of kills. Instead using the pistol from afar is the best option always aiming for the head of the Infected players. Once the numbers begin to drop in your group of non-Infected players it is time to switch tactics. This is the part of the game where most of the kills are made for a single player. The shotgun becomes paramount to your survival so spare the ammo as much as you can. Hopefully the opposing Infected players will attack sporadically (most likely) which should make it very easy to pick them off one by one from your vantage point. When choosing a vantage point make sure that your back is against a wall or is inaccessible. The corner of a level is a good place as your opponents will have little chance to catch you by suprise. Once set enjoy racking up the kills.
As an Infected player it often gets frustrating, especially when there is no order to the attacks oon the non-Infected. For this reason it is advised to work together as much as possible, whilst also taking advantage of an opening that one of your 'allies' provides. Invisibilty, where applicable, is a great tool at your disposle. Use it wisely and you will most likely kill several opponents with one invisibility orb. being the infected it is likely you will not get a large amount of kills. This means that the best approach would be to attack the players away from the main group first, as they are easier targets. Once the non-Infected group begins to decrease take it slower and try not to rush into the area where the opponents are located. Whilst being ann Infected player the main thing to keep in mind is timing.
Related ThreadsInfection Tips and Strategies:Now with an Index - last post @ Sep 22, 2012
Infection Spawns - last post by @ Jun 16, 2011
Flood Mode: The New Infection? - last post by @ Oct 9, 2012
Best catagory for infection? - last post by @ Oct 24, 2008
Infection or Invasion? - last post by @ Nov 4, 2010