Carrier forms are formed from older Combat Forms or from bodies that are unsuitable for combat. Lieutenant McKay theorized that Unggoy and Kig-Yar couldn't become Combat Forms and instead directly converted into Carrier Forms, though this theory has been disproved.
Combat Forms can be converted when they reach the end of their life span, or become too damaged to fight. Carrier Forms perpetuate the Flood species by acting as incubators for newly created Infection Forms and keeping them safe until they're fully developed. Eventually, when a Combat Form has lived long enough, it will evolve into a Carrier, regardless of its status.
Once the Infection Forms are ready, the Carrier simply explodes spontaneously, sending the new Infection Forms flying into the air. As soon as the Infection Forms are released, they begin searching for new hosts. It should be noted that Carrier Forms won't wait until full maturation has occurred.
If potential enemy hosts are nearby, it will simply get close to them and explode, sending the Infection Forms flying at the unsuspecting and often injuring or killing enemies. The explosion of a Carrier Form is similar to the explosion of a fragmentation grenade, and deals damage to anything nearby.
The Carrier Form has a simple mindset, which is to release the Infection Forms it carries to a suitable host. Other than organic bodily sounds, they cannot speak or make any other sound. Though they can somehow communicate with other Carriers, possibly through a Gravemind or Brain Form, to inform them of a potential sentient being.
Carrier Forms are usually considered slow and of little threat unless they are close to the player. However, one particularly dangerous trait of exploding Carrier Forms is the explosive chain reactions they can cause. A Carrier explosion has a similar effect to the Fragmentation Grenade, and as such can knock around objects such as crates at the player indirectly, either lowering their shields to a dangerous state or if wounded enough, kill them. Like grenades, they will hurt allies and a well prepared player can easily kill one to set off a reaction to destroy nearby Combat and Carrier Forms. This effect can be used to great effect in Halo: CE as the explosion radius is large enough to send nearby Combat and Carrier Forms flying across the map. In Halo 3, this effect is lessened and may not help in certain situations.
Any direct explosion eliminates the threat of the Carrier, as well as any Infection Forms that were incubating inside of it. This means that enough needles from a Needler will kill it, or you can throw a sticky grenade at them.
Carrier Forms are one of the easiest enemies to take down. One shot from any weapon, say the Plasma Pistol, will force it to explode. However, there is not a single weapon that can actually blow it up, not even a direct hit from a Rocket Launcher. Therefore, this has to be taken into account, strategically. For example, when confronting a large number of Flood, you may wish to destroy the Carrier Forms, which in turn will kill all nearby Flood. First, allow the Combat Forms and other to get reasonably close. Then, use any weapon - preferably one which has ample ammunition - to kill it.
It is strongly advised not to use Sniper Rifles against the Flood; it is simply a waste of valuable ammo. In any case, Sniper Bullets simply pass through Carrier Forms, dealing virtually no damage.
In Halo: CE a good tactic is to stick one with a Plasma Grenade and shoot it with a Shotgun; the Shotgun will cause the Carrier to blow up and the Grenade will kill the Infection Forms inside. Alternatively, you can just stick the Carrier with a Plasma Grenade. Make sure to aim for the "head" or above.
Running over Carrier Forms, on the other hand, is a very bad idea. Their explosions will easily flip your vehicle, wasting precious time. In Halo: Combat Evolved, you will also be severely injured.
- In Halo: Combat Evolved, the level "The Maw" contains a glitch in which the Carrier Form explodes, but the Infection Forms do not move.
- The Carrier Form is the successor of the Simulacrums, from Marathon. The Simulacrums were cybernetic humans made to look like the crew of the U.E.S.C Marathon, the only flaw was that they all wore green jump suits, making it easy to distinguish which humans were actually Simulacrums. As soon as you were three meters close to them they would explode, heavily damaging or killing the player.
- Carrier Forms in Halo: Combat Evolved and in Halo 2 are often distinguished from their Halo 3 counterparts in the way they explode. The ones in Halo: Combat Evolved and 2 will play a death animation before exploding, and there is no way to kill them or make them explode without making them play it. In Halo 3, however, they are rigged to explode when their health reaches zero, rather than play the death animation then explode, although they could still drop to the floor and detonate at will.
- Strangely, when shot by the Flamethrower, the Carrier Form will explode and release the Infection Forms, which seem to survive the fire. It is possible that the Carrier Form's explosion puts out the fire. This is strange considering that an Incendiary grenade will kill the Carrier and the Infection Forms.
- This could be because the Incendiary grenade has a small explosive force used to scatter the flames, which would kill the infection forms.
- It is also possible that the liquid fuel in the Incendiary grenade stop the flames from being blown out by the Carrier form's explosion.
- In Halo: Combat Evolved, a Carrier form can produce short bursts of speed once it spots a target.
- In Halo 3, a Spartan Laser beam can only penetrate two Carrier Forms. In turn, this will make the Carrier Forms explode without releasing any Infection Forms at all, similar to sticking a Carrier Form with a grenade or killing one by a Needler supercombine explosion.
- The Carrier Forms in Halo: Combat Evolved and Halo 2 would release between 4-6 Infection Forms when they explode, but the Halo 3 Carrier Forms would release dozens, but this is leveled out by a smaller and less extreme damage radius.
- On rare occasions in Halo 3, the Carrier's arms and legs will be left behind upon death.
- In Halo 3, Carrier Forms can jump.
- There is a glitch that if spawned, the Carrier Form will do nothing, and just stare at you, even at close range.
- In Halo: The Great Journey: The Art of Building Worlds, artist Robert McLees says that the carrier form was based on his cousin's infected thumb.
- Halo: The Flood, page 239: The Spartan (...) sidestepped to evade a charging carrier form
- Halo: Combat Evolved, level The Library
- Halo 2, level The Oracle
- Halo 3, level Floodgate
- Halo: First Strike, page 47: The Infection Forms rewrite a victim's cellular structure and convert him into a Combat Form, then later a Carrier Form
- Halo: The Flood, page 271
- Halo Wars
- Halo: Ghosts of Onyx, page 197
- Halo 3, level Cortana