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.

Transformation
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. 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. Some Combat Forms are equipped with Energy Shields or Active Camouflage.

In The Oracle the combat forms can be seen climbing up the walls with spider-like abilities.

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.

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.

Killing 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 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, has something actually 'hitting' it. In Halo: Combat Evolved the most effective weapons to bring down the Flood are the Shotgun and the Assault Rifle. The Shotgun is the best weapon for killing Combat Forms, but at longer ranges the Assault Rifle works the best. However, the M6D Magnum has also been found useful against Combat Forms (2-3 shots to the upper chest) and the Carrier Form (the pistol's zoom allows for the Carrier to be neutralized at a distance, possibly damaging other flood) in Halo: CE.

Making Flood Friends
In Halo 2, if you take away their arms, the body will disappear and the Flood Infection form comes out, but in Halo:CE, you can neturalize 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.

The Flood Combat Forms have also been known to completely forget about their target and move onto another. While this is quite annoying if it happens to you, it can be quite useful if they move on to create a distraction against a large group of Covenant/Sentinels. The reason why they do this can either be a demonstration of the Flood's tactics in fighting, or just because of the game AI's intelligence.

SubForms
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. It is unknown what happens to Brutes when infected by Flood. 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), 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. In Halo: Combat Evolved they do not have the capability of using Covenant Energy Shields, though they do, on occasion, use Active Cloaking Devices. In Halo 2, some Elite Forms use energy shields and cloaking devices (only on Legendary) or if you activate the "Envy Skull". Fortunately, because of Elite physiology, they are 'bigger' than the human form, therefore presenting a bigger target.

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. They seem to drive vehicles more often than the Elite form as well.

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 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.