Combat Form

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

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 strong host or 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 grows 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) and pilot any Vehicle.

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.

Some Combat Forms are equipped with Energy Shields or Active Camouflage probably because the host's body was not damaged too much when they were killed by a combat form or something else which sometimes might allow the technology to still work in their armor.

In the level The Oracle, the combat forms can be seen climbing up the walls with spider-like abilities and can even be seen wall jumping. The flood pause before jumping each time and do it with a stronger push of their whole body instead of just their feet.

Known Types of Combat Forms
Hunters cannot be infected by the Flood as they are not one sentient being but many, however while the other Covenant species are known to be infected in the books, only two known sub-forms are seen in the Halo games. Brutes have never been seen infected by the Flood, possibly because their thick hide prevents the infection form from entering the body cavity. 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)and in Halo 1, they sometimes use their dangling head as a weapon too. They also have the ability to wield both human and Covenant Weapons. It is very fast, very strong, and can jump great distances. Elite Combat forms can also jump 15 meters (almost 50 feet) in the air. Due to the Flood Controller's indifference to its host's discomfort, it is nearly immune to some weapons (notably the sniper rifle), but reliance on the host life support systems render it vulnerable to shotgun blasts. In Halo: Combat Evolved they do not have the capability of using Covenant Energy Shields, though they do, on occasion, use Active Camouflage Devices. In Halo 2, some Elite Forms use Energy Shields and (on all difficulties). Fortunately, because of Elite physiology, they are 'bigger' than the human form, therefore presenting a bigger target. Flood Elites have also been know to climb up walls when knocked out of the map.

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 exactly as much damage as the Elite Combat Form can, it has some advantages to its counterpart, It is more rapid and 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 but in Halo 1 it is slower. It shows a preference for human weapons(due to the host 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.

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 Infection Forms.

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. Explosive weapons such as grenades, Rocket Launchers, and both the Brute Shots grenades or blade can instantly kill a combat form. 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 longer ranges the BR55 Battle Rifle, Covenant Carbine, or the M6D work the best against Combat Forms (2 shots to the upper chest). At close to medium ranges the M6C Magnum and SMG are also very effective against combat forms due their fast fire rate. You may consider hitting the deceased combat form with a blade or grenade. It will then not be revivable, as it is destroyed. The Sentinel Beam is also an effective weapon due to its continuous fire and burning capabilities.

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:CE, 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.

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.
 * 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 Quarantine Zone level 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.
 * ocasionally if you throw a plasma grenade on a combat form it will charge at the player or any other non flood targets.