Flood combat form

A Flood Combat Form is an organism suitable for combat that has been infected by the Flood parasite via an Infection Form or Spores. The victim is mutated into a combat unit that fights for the Flood. They coordinate attacks on certain enemies by a Gravemind, if there is one.

Pathology
A Combat Form is not a natural organism, but rather a mutated host infected by parasitic Flood Super Cells implanted by an Infection Form. Successful creation of a Combat Form requires a host with sufficient biomass and calcium store. After mutation begins, the infection form attaches with a protruding neural connection that allows it to command the host's nervous system, thus rendering command of the legs and arms useless. 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.

The host stays alive during and after the transformation process and can be conscious and aware even after the final transformation. The victim is kept alive after infection so that it lasts longer as fuel for the Infection Form. This process is presumably very painful and terrifying. Once fully transformed, the Infection Form has total control over the host's body and changes the physical appearance of the host to better suit its own needs (although an infected host will be roughly the same size as it was before). On rare occasions, this effect may be weakened by time in stasis, allowing the host to regain some control, as in the case of Private First Class Wallace Jenkins. Combat Forms retain the host's previous skills and so can wield weapons, though cannot dual wield, drive vehicles and board enemy vehicles. However, Infection Forms do not require their hosts to still be alive for infection. Even dead hosts are suitable for conversion into a Combat Form, reanimating again after being infected; though the brain tissue remains dead, the victim's biomass and calcium reserves are sufficient to warrant infection.

In the initial stage of a Flood outbreak, the Feral Stage, Combat Forms communicate using pheromones and have one instinctual behavior; to secure new hosts to strengthen the local controlling intelligence. Once a sufficient store of biomass is available for the Flood to enter the Coordinated Stage and begin forming Pure Form Flood, the Combat Forms are used either as defensive units or calcium/biomass reserves. According to the Master Chief, an Elite Combat Form looked like an Elite, except it appeared that someone had "killed him, buried the body, and dug it up two weeks later". Flood infection is a rapid and efficient biological process, which can take under a minute in extreme cases. An apparent side effect of the deadly Human disease Boren's Syndrome, which Sergeant Avery Johnson suffered from, is total immunity to Flood Super Cells. The Flood can recognize if a potential host is immune, and will focus on that organism above all other threats in the area. A group of Flood will occasionally go after Sentinels if they are perceived to be the greatest threat in the area, although Sentinels are unable to be infected by the Flood. Curiously, in Halo 2, if there is a single Flood left that matches the species of the player (i.e. Human Flood if playing as Master Chief), the Combat Form will not attack the player, but simply stand idle, implying that the memories the Flood has absorbed seems to have a degree of influence over their actions. However, if other Flood arrive, it will resume attacking.

Types of Combat Forms
A specific creature is needed for the infection and creation of a Combat Form. Of the various Covenant races, only a few species meet the necessary requirements. Species known to be immune to conversion include the Hunter, whose complete lack of skeletal calcium (i.e. bones), colonial physiology and insignificant biomass per individual worm make infection and neural synchronization impossible for Infection Forms, the Drones have an exoskeleton therefore lacking bones making infection impossible and the Engineer is a 'gas based' life form and therefore believed to be immune.

Human Form
The Human Combat Form is significantly smaller than other forms due to their original smaller size. 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 fight. Although it cannot take as much damage as the Elite Combat Form can, it has some advantages.

It seems to be more alert and is quicker to melee an opponent. It also presents a smaller, harder target to hit, especially the Infection Form, as it isn't protruding out of the body as much as those of Elite Forms. However, in Halo 3, the Human form becomes an easier target to hit; the Infection Form protrudes a lot more out of the host's chest, allowing for an easy headshot, and it seems to be significantly slower.

They wield the weapons of former UNSC Marines and, if available, those of the Covenant. On a rare occasion, if the player leaves a detached turret next to a dead Combat Form, when the Combat Form gets reanimated it will wield the turret. They can be dangerous when wielding UNSC weapons like the Shotgun, which on higher difficulties can kill you in a couple of shots. In Halo 2, they are also capable of driving vehicles such as Warthogs, Scorpion Tanks, Ghosts, and Type-25 Assault Gun Carriages. While they lose the ability to drive vehicles in Halo 3, slow moving ones can still be boarded. They can wield any weapon, from Battle Rifles to Brute Shots. But like the Brute Combat Form, they only use their tentacle arm to attack. In Halo 3, regardless of the Marine's gender, once infected, the Combat Form will be male. Once in a while you may also find one with a rocket launcher. These Rocket-Wielding Flood are rare in Halo 3, and only appear every so often in Halo 2, but there always seems to be one at choke points (e.g narrow corridors) in Halo: Combat Evolved.

During Infection, the host will twitch around in pain, and eventually collapse. Afterwards, the whip tentacles will come out of his or her left arm, and the Infection Form will come out of the chest, where it entered the host.

Sangheili Form
During infection, the Elite's head is forced backwards, to create space for the Infection Form to reside. This Combat Form is a formidable enemy and is capable of deadly physical attacks using whip-like tentacles. It can also wield both Human and Covenant weapons.

It is very fast, very strong, and can jump great distances and heights like most other Flood. Some Elite combat forms can still use their armor's Active Camouflage and Energy Shields. Fortunately for the player, because of Elite physiology, they are bigger than the Human form and present a larger target to hit, but they will go down quicker if you aim for the parts of the torso without armor covering it.

All of the Elite combat forms encountered, have the blue armor of a Minor Elite in Halo 2, regardless of the hosts actual rank and original armor color. In Halo 3, they have the same color armor as the host. The normal armor color you will see on an Elite Form in Halo 3 is Blue, Red, and sometimes White. If the armor color is not blue, they are considerably harder to kill with melee, taking as much as 10-20 hits before they die, unlike the usual 2-4 hits. They can use any vehicle in Halo 2, but like the others, they can't use any in Halo 3, but they still can board a slow moving one and hit a devastating blow. If equipped with a Gravity Hammer it will use its tentacles instead.

In Halo Wars, Elite combat forms' heads are not forced backwards. This may be an oversight on Ensemble's part, or it may be there to make the Flood scarier.

During Infection, the infection form will go into the Elite's chest. Afterwards, the elite's back will start to swell with Flood Biomass. Then, the Elite will collapse onto the floor and be turned into a Combat Form.

They are known to use many weapons from the Type-25 Carbine to the Energy Sword.

Jiralhanae (Brute) Form
Brute Combat Forms behave much more aggressively in combat towards an opponent. It charges at an opponent much more than normal Brutes.

During infection, the Infection Form burrows into their skulls and forces the jaws apart to create a residual space. The creature seems to draw on its host's brutish nature and often forgo projectile weapons in favor of close combat. They usually wield Brute weapons such as Spikers, Maulers, Brute Shots, and even Gravity Hammers, as well as more traditional Covenant weaponry and Human armaments.

However, they do not use the Gravity Hammer to great effect, they just use it to melee instead of using the primary explosive hit (though that may have been a consideration on Bungie's part, it could be possible that they will kill themselves in the shock wave the Hammer produces). They are more powerful than the Human Form, but they are slower (bulkier) and can't jump as high due to their weight. Since when a Brute's armor is damaged far enough, it falls off leaving a stripped body, when a Flood Infection Form infects them, they don't have shielding capabilities like the Elite form. A shotgun round or a few melee attacks should be able to kill one.

Grunt and Jackal Combat Forms
Although they do not appear in the trilogy, Grunt and Jackal Combat Forms are introduced in Halo Wars. While no canon source ever denied it was possible for Grunts or Jackals to be converted, it was thought that they were unsuitable for conversion due to their small size and an assumed lack of calcium. Instead of using Grunts and Jackals as combat forms due to their size, The Flood mutated them into carrier forms. This assumption came from conclusions made by the fanbase prior to Halo Wars' release.

Their introduction is representative of the Flood's malleability - any creature of any type can be used by the Flood for its purposes. It is also be possible that these forms are just Grunts and Jackals which have Flood spores in their bodies, and would later be used as material for the Flood's biomass reserves. Another possibility is that they are freshly infected hosts prior to being converted into Carrier Forms or a unique case present on the Flood-controlled Shield World.

Anti-Flood Countermeasures
Combat Forms can take extensive and sizable amounts of damage before being neutralized. Arms, legs, even heads can be totally blown off before they are incapacitated. The weak areas of a Combat Form are its sensory appendages that normally develop on the upper chest. This is where the Infection Form has nestled, and the body is incapacitated if this area is destroyed by a projectile weapon.

In all three Halo games, a Combat Form may fall only to stand up again and keep fighting. This can only happen if it still has a usable limb and appendage. In Halo 2 and Halo 3, 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. Human weapons are generally more effective against the Flood. Covenant plasma weaponry is designed to debilitate by causing burning pain and structural damage to bodies, but the Flood cannot feel pain or be crippled. Piercing weapons such as the Sniper Rifle have little effect on combat forms, however, they can be killed with sufficient force from projectile and melee weapons.

Explosives such as grenades, rocket launchers, and the Brute Shot can easily kill a combat form. Plasma or Spike grenades should be used with caution, as the Combat Form will sometimes ignore all other targets and charge players using those weapons at high speed.

The Needler can be devastating, as the seven-needle explosion will prevent it coming back to life in Halo: Combat Evolved, and will shatter it in Halo 2 and 3. In addition, the Needler's explosions will destroy an entire horde of charging Flood Combat Forms, similar to grenades and rocket launchers. However, for this to work, you must have at least 7 needles embedded into the Flood form. At close range the Shotgun, Mauler, Brute Spiker, Brute Shot (melee only), Gravity Hammer (due to its 'explosive' area effect shockwave) and Energy Sword are the best weapons.

At close to medium ranges the M6C Magnum and SMG are fairly effective against Combat Forms due to their fast fire rate. The Sentinel Beam is also effective due to its continuous fire and burning capabilities. At long range the BR55 Battle Rifle, Covenant Carbine, or the M6D can be used to fire two shots to the upper chest. Shooting the arms off Combat Forms is effective in Halo: Combat Evolved, using this is known as the "Flood Buddy" glitch. The Form will not be able to attack you and will just follow you around. A single Combat Form can be good target practice with the M6D using this method.

Burning Flood forms is one of the most effective methods in any situation, as is destruction of the torso. However, if you burn the Flood form, and it is not dead, it will charge toward you, burning, and damage you as well. In Halo 2, the Sentinel Beams dropped by the Forerunner Sentinels are also very effective against the Flood. In Halo 3, plasma and melee attacks, formerly weak against the Flood Combat Forms, are now highly effective: a Combat Form can usually be destroyed with 1-3 melee attacks depending on its type and the difficulty setting, and weapons such as the Plasma Rifle are very effective, especially when dual-wielded. Also, a melee attack with any weapon to a Combat Form corpse render it unusable to Infection Forms. This makes gameplay easier, as, previously, it took around 7-10 melees to kill, even for assassinations.

General

 * In Halo: Combat Evolved Combat Forms will not disintegrate from successive melee hits when they are dead like in Halo 2 and Halo 3.
 * In Halo 2 and Halo 3, while Marines cannot wield weapons such as Energy Swords, Brute Shots, and Sentinel Beams, Human Flood have the ability to wield them. This could be that while normal Humans do not have the knowledge or skill to wield such weapons, Human Combat Forms do possess that knowledge.
 * In Halo 2 and Halo 3, Flood Combat Forms retain more of the host's armor, such as chest plating. They were almost naked in Halo: Combat Evolved. This could be attributed to the Flood becoming more intelligent, due to the Gravemind's influence.
 * Flood Combat Forms seem to take on physiological and biological changes throughout the Halo trilogy, these changes from game to game could be due to the Gravemind's influence in attempting to create a better Flood soldier.
 * Occasionally, if a player throws a Plasma Grenade on a Combat Form, it will charge towards the player, ignoring any other non-Flood targets. In Halo 3, they will jump away from grenades or (unsuccessfully) attempt to shield themselves from the resulting explosion.
 * A very ineffective weapon to use against them is the Sniper Rifle and Beam Rifle due its high velocity shots going straight through the Combat Form.
 * The Flood, unlike most other enemies in the games, do not instantly die when meleed in the back. This is most likely due to the fact that they don't have backbones.

Halo: Combat Evolved

 * In all of the Halo games, Flood Combat Forms always hold the Shotgun with one hand, and fire it without pumping another round into the chamber. In Halo: Combat Evolved, they somehow manage to make the shotgun pump itself. A possible explanation for this is that the Flood has extended an appendage into/onto the Shotgun which allows it to use the Shotgun's pump as a body part. That, or the pump action just wasn't programmed into Halo: Combat Evolved.
 * Out of all weapons the player can use, the Sniper Rifle is the only weapon Combat Forms cannot wield.
 * In Halo: Combat Evolved if you stand on a Light Bridge (343 Guilty Spark) and shoot off armed Combat Forms' gun arms, they will leap onto the bridge and get stuck and will only be able to move by jumping at you.
 * If you headshot Combat Forms (usually human) with a Pistol, half their face will get blown away, and they will fall but will most likely stand up again.
 * If you shoot both arms off a Combat Form in Halo: Combat Evolved it will not be able to attack you and will simply follow you around. Players have dubbed this instance as creating a 'Flood Buddy'. On rare occasions, the Flood will hit you with its head once and then become harmless.
 * In irony, Elite Combat Forms will wield human weapons far more than Human Combat Forms; They often make use of the MA5B Assault Rifle. The reason might be that there were more Elites than Humans during the Battle of Installation 04.
 * Elites using the MA5B have been seen firing it in bursts of about two shots per pull.
 * This is the only game in which an Infection Form cannot revive a dead Combat Form; instead there is a chance that the Combat Form will just get back up again.

Halo 2

 * Halo 2 is the only game where the Flood use vehicles (in the level Quarantine Zone).
 * During some parts of the Halo 2 levels with Combat Forms some are seen rarely using Sniper Rifles, although their poor aim suggests that the form's feeler-based sensory would make using scope-based weapons pointless (when they use the M6D pistol in Halo:Combat Evolved they always aim for the chest). On Legendary difficulty, however, these Combat Forms are incredibly deadly as they can kill the player with one shot.
 * In Halo 2, all the Flood infected Elites wear the armor of an Elite Minor, even on levels where no Minor Elites were found. In Halo 3, an infected Elite Ultra retains the appropriate armor of an Ultra, but reverts to that of a Minor when it is killed.
 * Sniper Rifles are very ineffective against any Combat Form, Human and Elite, as it will just go through them. However, it can be successfully used to hit their "weak spot"; This proves to be fairly difficult because the dot sight that comes with weapons such as the Battle Rifle is absent.
 * The Energy Sword's energy drops only a little when used against the Flood, about 2.5 per lethal hit. This makes it a successful anti-Flood weapon.
 * When using the Grunt Birthday Party skull against the Flood in Halo 2, the effect of the skull will cause the plasma grenade explosion to occur when you shoot the Combat Form in the chest, where the Infection Form is.

Halo 3

 * The Combat Form can pick any weapon should it be near them when they "resurrect."
 * Sometimes, Flood will wield Flamethrowers if they are revived close to one. In some cases, they can be seen wielding Plasma Turrets with one arm or tentacle. This is common on the level The Covenant, right after you kill the Brute Chieftain with the Plasma Cannon. This may either mean that the Flood has built up a powerful muscular structure in the host during infection, or they haven't been programmed to hold these weapons with two hands.
 * A Combat Form's old "head" (the one that used to belong to the host) will move or twitch as if in pain if you watch it in theater mode.
 * Combat Forms can pick up weapons discarded by the player, i.e. Spartan Lasers, if there is any ammo/charge left in the weapon. Care should be taken when dropping these kinds of powerful weapons around Flood forms, as they might be used against you by revived Flood.
 * An infected Chieftain will turn into a Brute Combat Form with Minor Brute armor on. These errors were due to memory constraints and also represent the rapid increase of visual effects technology over the course of the development of the games.
 * Brute Combat Forms can hold Gravity Hammers and Brute Shots with one hand, although they rarely use a Gravity Hammer.
 * If an Elite is infected, a Gravity Hammer-like pulse will be emitted from his suit. It is unknown why this happens, though it could be a remnant of an energy shield, or simply a glitch.
 * The Gold Skull "Fog" is held by a Flood Combat Form on the level Floodgate.
 * Halo 3 is the only game where you can instantly kill a Combat Form by meleeing them with something other than a Energy Sword, Gravity Hammer or Brute Shot.
 * It is possible to see and shoot the Infection Form in the host's chest. If you successfully destroy the Infection Form and leave the empty corpse intact and "face up," you will see a large purplish hole where the Infection Form was. When an Infection Form "resurrects" the corpse, the Infection Form will inhabit that previously mentioned empty cavity.
 * Regardless of gender, an infected Human Flood becomes a male Combat Form. There are no variations between an infected male or female Marine.
 * Rarely, in the level The Covenant, a Brute will appear that is only half infected, with its head still visible.
 * On occasion, a Combat Form will appear that is immune to sword melees. This is most often seen in Cortana.
 * If you manage to kill Elites sporting an Assault Harness mere seconds after an Infection Form enters it, the transformation will be canceled out and the Elite in the Assault Harness will become a Minor Elite in a Combat Harness.