Combat Form

A Combat Form is a vertebrate suitable for combat that has been infected with the Flood parasite via an infection form and mutated into a combat unit that fights for Flood goals and objectives.

Introduction
To become a combat form, one has to be infected by the Infection Form. If the Infection Form obtains a frequency match for the host's neural system, it begins to rewrite the very DNA of its host. While in most cases, the Infection Form succeeds in infecting its host, occasionally, a match isn't found, and the being can't be infected, as was the case of Sergeant Johnson.

Once fully transformed, the Flood infection is usually in total control of the body, depending on the strength of the Infection Form and the strength of the host. A weak infection form allows for the host to gain some control over their bodies. Unfortunately for the host, he/she stays alive for the duration of the transformation process and is even conscious of his/her environment as the Flood take over the body.

Once completely infected, the host becomes a Combat Form. Any body capable of carrying a weapon or one that is still useful in combat becomes a Combat Form and is very deadly. The Combat Form sprouts whip-like tendrils on its arms, and will use them as melee weapons in close range. These are created with the host's calcium stores. The Flood Human and Covenant Elite Combat Forms have been seen and known to use any weapon (except the Fuel Rod Cannon). They can also pilot most Vehicles and board enemy vehicles.

The Flood infection is capable of not only controlling the host's body, but of also tapping into its memories to find relevant information about using weapons and equipment. In Halo 2, the Flood have learned how to tap into the host's knowledge of how to drive vehicles and aircraft.In Halo 2 a combat forms' speed is slightly faster than a ghost moving backwards.

Known Types of Combat Forms
Hunters cannot be infected by the Flood as they are not one sentient being but many. Jackals cannot be infected because they do not have enough calcium (hollow bird bones) to sustain the infection form. Grunts are turned into Carrier Forms. Drones cannot be infected because of their exo-skeleton. Brutes have thick hide/fur. They do not just want combat prowess, but intelligence and memories, as well. There are two following seen Combat Forms in the Halo series:

Elite Form
The Covenant Combat Form is a formidable enemy; it is capable of a deadly physical attack using its tentacles (once the arms of the Covenant host). They also have the ability to wield both human and Covenant Weapons. It is very fast, very strong, and can jump great distances and heights. During the Infection process, the host's internal organs are liquefied and the nutrients from them are used to develop the tentacles and other appendages of the combat form. Because of this, many weapons (Notably the sniper rifle) have little to no effect against the flood. Also, physical attacks (Aside from with a Plasma Sword, Brute Shot, or Gravity Hammer) do not have a big effect, reliance on the host life support systems render it vulnerable to shotgun blasts.However a weapon that is heavy in weight has a much stronger melee that can be effective if the player can avoid their lunges. This includes the the Rocket Launcher, Fuel Rod Cannon, and even the Sentinel Beam. Some Elite combat forms can still use their armour's Active Camouflage and Energy Shields. Fortunately for the player, because of Elite physiology, they are 'bigger' than the human form, therefore presenting a bigger target. Flood Elites have also been known to climb up walls in the level the Oracle.

Human Form
The Human Combat Form is significantly smaller than the Covenant version (due to the comparative differences of the host bodies). Although it cannot take in as much damage as the Elite Combat Form can, it has some advantages to its counterpart, It is a bit more alert, it melees much faster, and the infection form is harder to shoot because the Human host is smaller than an Elite. It has the same speed as the Elite Combat Form in Halo 2. In Halo 1 it does not burst into speed as often as the Elite Form. It shows a preference for human weapons (due to the hosts' memories of using them more often), and can be deadly with a shotgun in close quarters, and the most deadly with the M6D at any range. As with other Combat Forms, it is capable of sustaining massive physical damage before being unusable to the Flood Controller; in fact, as long as the chest cavity and legs remain intact, it will continue to try and fight. They seem to drive vehicles more often than the Elite form as well.

Brute Form
Not much is known about these new combat forms, other than they are Brutes that have been consumed by the Flood.

Combating a Combat Form
The Combat Forms can take extensive damage before being neutralized. Arms, legs, even heads can be totally blown off of their bodies before they are incapacitated. The only known weakness of a Combat Form is its sensory feelers that normally develop in the upper chest region. This is where the Infection Form has nestled, and once destroyed by a headshot-capable weapon, the body is incapacitated. In Halo and Halo 2, a combat form may fall, only to stand up again and keep fighting moments later. This can only happen if it still has a usable limb. In Halo 2, if the body is not destroyed, such as by an Energy Sword, Grenade or Brute Shot blade, Combat Forms can be "revived" by other Infection Forms crawling into it. Also Note that if you shoot into the sensory feelers in the upper chest region while a headshot mark appears in the reticle with weapons that actually can do headshots (almost exclusively human weapons) while the "Grunt Birthday Party" skull is in effect, the flood will explode and be completely destroyed, or, in the case of the flood surrounding the "Famine" skull, be blown into the air and return to the ground completely unharmed and unmoved.

Explosives such as grenades, Rocket Launchers, and the Brute Shot can easily kill a combat form. In both games, the Flood Combat Form is more vulnerable to human weapons, which unlike the Covenant small arms with the exception of the Needler, has something actually 'hitting' it. At close range the Shotgun and Energy Sword are the best weapon for killing Combat Forms. A shotgun blast at short range or sword lunge will take them out permanently with no chance of them reviving. At close to medium ranges the M6C Magnum and SMG are also very effective against combat forms due their fast fire rate. The Sentinel Beam is also an effective weapon due to its continuous fire and burning capabilities. At longer ranges the BR55 Battle Rifle, Covenant Carbine, or the M6D work the best against Combat Forms (2 shots to the upper chest where the flood infection form is).

Making Flood Friends
In Halo 2, if you take away their arms, the body will explode and the Flood Infection Form comes out, but in Halo: Combat Evolved, you can neutralize the Combat Form by blowing off its arms, head and even part of its stomach. This harmless torso on legs will follow you and can even be used as a shield against enemy fire.

End of Flood Life Cycle
Eventually, a Combat Form will mutate into a Carrier, regardless of damage, to perpetuate the Flood species. The Combat Form will prematurely transform into a Carrier Form if it takes enough damage and the Combat Form cannot function effectively. As a Carrier, the host will basically become a walking incubator for more Infection Forms.

Halo 3
It is known that the Flood will return in the third game of the story arc. We are hinted that they will somehow get to Earth.

Due to the 360's more powerful graphics engine, it is possible that the Flood will look slightly different(hinted to be more like they were in the graphic novel) although we can't know for sure yet as they have not been shown yet, and likely won't be shown until the game is released. They could have new abilities.

Trivia

 * In all Halo games, Flood Combat Forms always hold the Shotgun with one hand, and fire it without pumping another round into the chamber.
 * There is a glitch that when a combat form is holding a Flamethrower, (this is done by modding without tampering with the animation) it will raise it directly at the sky but the flames will still shoot towards you.
 * In Halo 2, all Flood Combat Forms have more of the host's armor left on them such as chest plating and etc.
 * In Halo 2, all the Flood Infected Elites wear the armor of an Elite Minor, even on the Forerunner Gas Mine where the only Covenant Elites are the Spec-Ops Elites and the Arbiter. Even in the entire level, Quarantine Zone and the level Sacred Icon are there only Minor Elite Flood, where no Minor Elites were found. This is probably due to make it easy to generate combat forms.
 * Occasionally, if you throw a plasma grenade on a combat form it will charge at the player, ignoring any other non-flood targets.
 * Strangely, they dont seem to be able to use the Fuel Rod Gun or Cannon.