After an Infection Form has obtained a frequency match for the host's neural system, it begins to rewrite the very DNA of the body it's using. Once fully transformed, the Flood infection is in total contol of the body. 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 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. The Flood Human and Covenant Elite Combat Forms have been seen and known to use any weapon. However, in both Halo: Combat Evolved and Halo 2 they have not been seen to hold the Fuel Rod Cannon, Sniper Rifle, or Beam Rifle. However there has been a glitch whereas a Human Combat Form in Halo 2 has been shown to wield a Beam Rifle, but this is a VERY rare occurance.
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 on the host's knowledge to drive vehicles. After enough damage has been taken and the combat form cannot function effectively, it becomes a Carrier Form that incubates more Infection Forms. Eventually a Combat Form will mutate into a Carrier, regardless of damage, to perpetuate the Flood species. Some Combat Forms are equipped with energy shields or Active Camouflage.
Also in Halo 2, if the body is not destroyed, such as by an Energy Sword, grenade, or shotgun, Combat Forms can be "revived" by Infection Forms.
Note: 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, the body is incapacitated.
If you take away their arms, the body will disappear and the flood infection form comes out (Halo 2) or it's a walking potato (Halo) -- See How to make Flood Friends.
The Flood Combat Forms have also been known to completely forget about it's target and move onto another, while this can get annoying if it moves onto you, it can be quite useful to create a distraction against a large group of Covenant/Sentinals. The reason why they do this can either be a demonstration of the Flood's tatics in fighting, or just because of the game AI's intelligence.
While the ability to blow off their arms and head is common knowledge in Halo, it is not common knowledge that you can blow off the Human combat form's stomachs in Halo 1.
SubForms
There are only two known sub-forms seen in the Halo games.
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), as well as having the ability to wield both human and Covenant weapons. It is very fast, very strong, and can jump great distances. 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. It does not have the capability of using Covenant Energy Shields, though it does, on occasion, use Active Cloaking Devices.
Human Form
The Human Combat Form is significantly smaller than the Covenant version (due to the comparative differences of the host bodies), and is both slower and weaker than its Covenant counterpart (though it has the same jumping ability). It shows a preference for human weapons, and can be deadly with a shotgun in close quarters. 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.
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