Flood

The Flood (otherwise known as the Parasite by the Covenant) is a species of virulent, parasitic organisms that thrive by consuming sentient lifeforms of sufficient biomass.

Introduction
The Flood are believed to be of extragalactic origin by the Forerunners, and their origins and purpose of coming to the Milky Way galaxy are unknown.

However, their potential for destruction is not in doubt. The Flood are an organism-scale parasitic species, and their sole means of reproduction are by assimilating other sentient species into their fold by neurological and physiological mutagenic means by entering and infesting live bodies or corpses. Their all-consuming nature and their dependence on sentient life means that it would be impossible for the Flood to be a part of any conventional ecosystem. As such, it is not unreasonable to assume that the Flood is not entirely natural if at all.

The Flood are believed to have arrived 100,000 years ago, when they were percieved as hostile by the Forerunners, and the Forerunners deployed the Sentinel drones to fight against them and attempted to contain them. Almost immediately afterwards, the Forerunners began a "Conversation Measure", researching the Flood, their adversaries, while constructing the Ark on Earth, protecting the "worthy" human race from destruction from the Flood. Some Flood were kept by the Forerunners for scientific study.

The Forerunners employed many measures in an attempt to contain the Flood, but ultimately failed. Finally, the Forerunners were forced to a pyrrhic solution, building seven ring-like installations, known as The Halos, across the galaxy. Because the Flood were parasitic, and their survival was immediately linked to the presence of potential hosts, the Forerunners reasoned that eliminating all potential Flood hosts, that is, all sentient lifeforms, would contain the Flood. After all other measures failed, the Forerunners activated the Halos, unleashing weapons of mass destruction across the galaxy, destroying almost all sentient lifeforms, including themselves. However, the human race, on Earth, was believed to be shielded by the firing of the Halos by the Ark construct.

The Flood were contained, lacking of hosts and thus unable to reproduce. The only surviving specimens were contained in top-class Forerunner research facilities, such as the Halo installations and the Threshold Gas Mine.

Biology
The Flood, while a joint parasitic species, its individual organisms undergo different stages of organized evolution, known as "forms". A Flood's stage of evolution depends on the species of creature infested, and the time the individual organism has had time to evolve.

The sole pathway for Flood reproduction and survival is by the infestation and assimilation of other species.

Flood Forms
The basic Flood form is the infection form, the smallest Flood type of organism and unique in its distinctive spherical shape. The infection form is used to infest organisms of other species, physically adhering to their surfaces and inserting tentacles, forcibly "hacking" into its host's nervous system, completely changing its psychology to that of the Flood's, and eventually physiologically mutating it into a combat form, carrier form, or prophet form, depending on the species of the host.

The combat form's responsibility is to kill all species hostile to the Flood: infection forms can infest other organisms regardless if they are living or dead. The carrier form's responsibility is to incubate infection forms, to spawn them within their body, and then disseminate them in active combat, exploding in massive detonations of gas and releasing infection forms into the fray. The prophet form's responsibility is unknown.

After a period of time, multiple combat forms may syncytically merge into a brain form, a larger, immobile creature. After a lengthier period of time, the syncytical fusion of Flood forms, with sufficient biomass, may evolve into the Gravemind form, a gargantuan Flood form that is the centralized sentient intelligence of the Flood, directing all lower Flood organisms.

Infection Form


The infection form is the most basic Flood form, and is the smallest, although it is believed to be the most important: it is the Flood's method of infecting and assimilating other species. It is a small, spherical organism, with a small, sagging, brown horn projecting from the top of its back, and tentacles projecting from its spherical surface. These tentacles propel the infection form through sinusoidal motion, and are used to bring the infection form towards its target. Upon reaching its target, it will attempt to infect it by extending its tentacles, penetrating its skin, and contacting the host's spinal cord, bringing about a successive series of neurological and physiological changes to the host. However, energy shielding provides some protecting against an infection form, popping the creature in a small, but damaging, blast of noxious gas.

However, the infection form's infectiousness decreases with time, and after prolonged millenia, their neurological control over a host weakens, allowing certain hosts to retain some consciousness seperate from the Flood psychology initially imposed by the infection form.

After infecting combat forms, the infection form resides in the chest cavity. When the combat form is destroyed, the infection form may or may not escape from the cavity.

Infection forms are extremely dangerous to unshielded enemies even in small numbers: a single contact between an infection form and an organism lacking a personal energy shield will lead to near-instantaneous assimilation within minutes. The addition of a personal energy shield is a tremendous advantage in fighting infection forms, making them dangerous only in significant numbers: the infection forms must first explode when reaching the potential host, lowering the shields with repeated detonations before seizing upon the unshielded host's armor, and then proceeding to assimilate, requiring a tremendous number of infection forms to breach the shielding and combat armor.

Combat Form


After an infection form has successfully latched onto a sentient creature with sentient intelligence, sufficient biomass, and is of combat potential, it will neurologically and physiologically mutate the host, twisting it into a combat form. Combat forms oftentimes maintain the general profile of the original host organism, and possess the original clothing and armor they were wearing before infestation. So far, infection forms have been seen only mutating humans and Elites into combat forms, not bothering to utilize Grunts nor Jackals as combat forms because of their physical weakness and lack of significant combat capability, instead turning lower creatures in Carrier Forms.

Combat forms are most damaging at close range, utilizing their whip-like mutated arms to whip scourging blows of significant damage, even stronger than a SPARTAN-II warrior augumented with strength-amplifying amorphous circuity. A single blow can lower a MJOLNIR Armor's personal shields by half, and few melee swipes are needed to drop a lowly Grunt or Jackal, while sufficient numbers can down even a Brute, Elite, or SPARTAN.

As mentioned below, combat forms can also utilize weaponry or vehicles, although they are poorly proficient with almost all firearms and are clumsy with vehicular operation. Combat forms are rakishly aggressive and lack even rudimentary battlefield tactics, although their sheer numbers can force even powerful enemies to succumb. Infected Elites or humans can be transformed into combat forms within minutes, and dead corpses can be alarmingly reanimated in seconds, making corpse destruction a useful tactic when combating the Flood, and is even recognized by the Forerunner Sentinels, who sometimes burn corpses to ensure that they can not be reanimated.

Carrier Form


When an Infection Form takes over a body that is unsuitable for combat, or once a combat form is damaged beyond use, it becomes a Carrier Form. Grunts and Jackals are immediately turned into this form upon infection, due to their weaker physiology, they are not deemed viable to become Combat Forms. Carrier Forms perpetuate the Flood species by acting as incubators for new Infection Forms, keeping them safe until they're fully developed, and disseminating them in combat.

Physically, they are a large, bloated sac situated upon a torso and a pair of waddling legs that slowly transport carrier forms. In combat situations, once they are within close proximity of a potential host or when their sacs are perforated even with a single projectile, systems within the sac form of the carrier form produce excessive amounts of voliatile gas, forcing the carrier form to its pudgy feet while the sac exponentially swells with gas for several seconds, then spontaneously detonating into fleshy scraps and hurling several infection forms that were spawned within the carrier form into the air. The detonation itself is extremely damaging, and its gas-propelled dissemination of infection forms facilitates easier assimilation, as the infection forms are likely to be hurled near potential hosts.

Brain Form
One of the rarest types of Flood is the Brain Form. A Brain Form is unlike all other Flood forms since it lacks any sort of mobility and is passive. It is believed to be the product of syntynctal fusion of approximately three combat forms, merging all of its subordinate forms into a massive, bloated spherical organism with spindle-like outgrowths that adhere to the surrounding environment.

Brain forms are unique in that the consciousness of the original host organisms are still vaguely maintained, greatly unlike most combat forms, allowing for slight manifestations of mental resistance to Flood assimilation. However, the Flood is able to, with high intuitive perception, sift through the individual memories of the original hosts, deleting erroneous ones while seeking memories that are of great importance to the overall Flood species. Once such a memory is found, it is incorporated into the general consciousness of the Flood species, and is believed to be incorporated by a Gravemind form into a general strategy for the overall species.

Prophet Form


Prophet Forms are highly rare products of infestation and assimilation of a Covenant Prophet. Because of the rarity of Prophets and their significant protection by other Covenant species and technologies, Prophet Forms are extremely uncommon. Many mechanisms and details of Prophet Forms are unknown, although it appears that the Prophet Form is directly infected and incorporated into one of the many kraken-like tentacles of a Gravemind form, and fully retains the original voice and full intelligence of the former Prophet.

The Prophet Form is like the Combat Form in the physiological distinctions it shares with the original organism before infestation: Prophet Forms can easily be compared to Prophets by their heads and to a less extent, their chests, merely having a central body of the Gravemind tentacle.

Juggernaut Form
The Flood Juggernaut is a large Flood form found encoded in the Halo 2 disc (the same way as the Engineers in Halo 1). The Juggernaut Flood is huge, at least twice as tall as the Master Chief. It can jump twice as far as the already ridiculously far jumping Combat Forms. It is speculated that the Juggernaut was created to destroy the Enforcers, as the tentacles would have no problem snatching one out of the air. It is also believed by some to be an infected Brute. There is nothing stopping it from being one, so it is possible.

Gravemind
At present it is not known what the Gravemind's purpose truly is. It is speculated that the Gravemind is comprised of a huge mass of flesh and bio-matter that Flood combat forms have to one place for absorption. It has a "mouth" through which it communicates, and has many far-reaching tentacle appendages.

It is theorized that the Gravemind may in fact control other Flood forms, the 'hive mind' of the Flood, or at least influences each form via telepathy. It is also speculated that the Gravemind may be a highly evolved Flood Brain form.

The Gravemind is made of countless bodies that were unsuitable as Combat or Carrier forms. The Art of Halo: Creating a Virtual World states that the Gravemind is “literally built from the bodies of its enemies and its own fallen warriors reassembled into a massive, tentacled, and intelligent entity."

It is not known how, or even if, the Gravemind seen in Halo 2 managed to survive the first Halo activation, as he(or it) being a sentient life form should have died with the Forerunners as well. Of course, it is unknown if the current Gravemind was even alive/formed when the Halo rings were activated by the Forerunners.

Infection


The Flood species's parasitic existence is fueled only by its infection and assimilation of other species, which is done by the Flood's infection forms.

Neurological
Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or dead bodies. Infection forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. From here they synchronize with the host, matching frequencies with the host's nervous system and taking over brain functions, completely rewriting the psychology of the creature to the ravenous and voracious psyche of the Flood, although retaining higher-level cognitive abilities, such as combat techniques and technical knowledge which may be useful. However, resistance of the host is completely destroyed, as the original mind's memories are systematically erased, leaving only a simplistic and animalistic hunger to assimilate other species and destroy them. During the early stages of infestation, when the host still retains its original consciousness, there are distinct somatosensory sensations: first the host's extremities grow numb and their lungs feel heavy. Then, the greater Flood intelligence begins to take over the mind, and sensation becomes increasingly undistinct: the host's vision seems to be "a gauzy white haze", but the host can make out shapes and the host can hear a low droning through the ordeal. Finally, once physiological mutilation occurs, the host can vaguely feel dramatic changes to its own body: the limbs feel to the host as if they are made of a spongy liquid and the lungs itch as well.

Physiological
While baseline neurological assimilation is same regardless of host species, the Flood's infestation includes physiological mutilation, which is species-specific.

Higher-level species capable of combat, such as Humans or Elites, are selectively infested to become Combat Forms. Combat forms retain the general profile of their original species, although they are significantly mutated. Organ-based systems physiology is corrupted, and organ-specific functions are decentralized and body cavities are decayed, making hitbox-selective incapacitation impossible: since organ functions are decentralized, decapitation does not down a combat form. Furthermore, highly area-specific munitions, such as sniper rifle projectiles, are completely ineffective, as picking off selective areas of a combat form leads to no wide-scale physiological impedements, and the projectile will punch through the decayed flesh rapidly, punching through and exiting the other side of the combat form without consequence. Combat forms also generate tentacles very quickly without regard for the skeletal structure of the host, with tentacles protruding from the combat form at odd locations, typically localized to the chest axial regions, such as the neck, armpits, or torso. A single arm is oftentimes mutated to a clawlike or tentacle-like structure that multifurcates at its terminus, with strange multiple angular hand-like entities replacing the hand. The musculature of the combat form dramatically changes as well, and the combat form is given prodigous arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. Metabolitic requirements, however, appear to be extreme on the organism: blood vessels become extremely prominent on the surface of the Combat Form, with thick and distinct bloodred-colored vessels spawning all across the creature's surface, most prominently so on the head.

Lower-level species with bodies uncapable for combat, like the Grunts or the Jackals, are more extensively physiologically mutated. Their lower extremities are mutated to become an extremely distinct torso and pudgy legs, while the upper parts of the body are horribly twisted into a large, bloated, sickly yellow-green sac with writhing tentacles projecting from the surface. Within this sac are systems believed to facilitate infection form spawning and also gaseous systems that, when triggered, cause the carrier form to explode into a conflagaration of highly volatile gas.

Countermeasures
Because of the millenia of Flood existence, it is believed that the Flood have evolved extensively so that there are little direct countermeasures against infection.

The only currently known way to effectively stop a Flood infection when a host is infected by an Infection Form is to have Boren's Syndrome, a neurological condition where radiological exposure causes electrical anomalies in the host nervous system. The neural frequencies of the host are scrambled, although without negative manifestations to the host. However, during Flood infection, the infection form is unable to rewrite the psychology of the host, and is unable to proceed to large-scale physiological mutation as well for an unknown reason, although Flood DNA is spliced into host DNA, even imparting the host with increased regenerative capabilities.

Behavior/Combat
While possessing an unusual sentient intelligence and combat forms still retaining extremely high-level cognitive functions such as vehicular operation, lower level cognitive abilities, such as precise dexterity and overall battlefield tactics, are notably lacking in the Flood, in their psychology and combat behavior. While the Gravemind coordinates Flood strategy on a galactic scale, small encounters are not coordinated between individual Flood warriors.

The Flood follow a zealous, ravenous strategy, with little target selectivity, aggressively attacking all hostiles in range. Furthermore, Flood tactics are based primarily on asymmetrical warfare: the Flood employ swarms of creatures, oftentimes outnumbering enemies by horrendous factors, compensating for their individual weakness with massive numbers in an attempt to overwhelm enemies and then reanimate their corpses afterwards.

The Flood have a strong preferability for close-range combat: combat forms possess abnormally strong melee attacks by whipping their tentacles, capable of dropping a SPARTAN-II's MJOLNIR Armor's shields in half with a single blow. Carrier forms cause tremendous devastation at close ranges, and their explosions are extremely devastating to friends and foes alike at close range, with the added benefit of disseminating infection forms in the immediate vicinity of enemies. Infection forms are capable of attacking only at point-blank ranges, exploding in damaging bursts when meeting a shielded opponent's personal energy shields or instantly grabbing hold of an unshielded enemy organism and infesting it. Combat forms can also use firearms, and have been seen clutching firearms in one of their hands (regardless of whether it is one-handed or two-handed). However, combat forms are extremely poor warriors at range with ranged weaponry: it is believed that the neurological ramifications of Flood infection significantly decrease accuracy, as well as weapon recoil if held in only one hand. The Flood are mostly only dangerous with heavy weaponry, such as the M19 Rocket Launcher or the M90 Shotgun, and are curiously ill-skilled with close-quarters weapons such as the Energy Sword.

However, as aforementioned, the Flood demonstrate a curious higher intelligence despite their poor combat aptitude and coordination. The Flood collectively learn the intelligence of any species of which even one member is assimilated, and selective hosts with particular knowledge, such as vehicular/starship operation, are still imparted with that intelligence after being infested. For example, during the ill-fated raid on the Infinite Succor, after even one Elite was assimilated, all the Flood could open any doors and access any system on the ship immediately afterwards, demonstrating that the Flood can share collective knowledge across their species. Furthermore, when the Flood then assimilated the Prophet Legate, the Flood reflected the high-level knowledge of the Legate through its individual organisms to Special Operations Commander Rtas 'Vadumee, mocking him with the words, "[t]he Forerunner not defeat us...what chance you?". The Flood, however, reflect unusual knowledge in the individual psychologies of its individual species in processes facilitating increased infestation: the Flood were sighted gathering bodies in the corners of the Covenant CCS-class Battlecruiser Truth and Reconciliation, perhaps making it easier for Infection Forms to find hosts to reanimate, and the Flood were gathering body parts for unknown purpose aboard the Infinite Succor.

Flood combat forms have been sighted crudely operating Ghosts, Warthogs, Wraiths, and Scorpions in ground operations, although with slow deliberation and slumsy driving and accuracy. The assimilated crewmen of the UNSC Frigate In Amber Clad were also able to initiate a Slipspace microjump for the frigate and perform rudimentary manuevering of the In Amber Clad, navigating the frigate from the atmosphere of Delta Halo to High Charity and then crashing the vessel into a tower, although whether or not this was from deliberate design or from clumsiness is unknown. In addition, the Flood can instantly absorb knowledge from an infected host into their collective mind.

The Flood's millenia of existence are believed to have made the assimilation process unusually brief: creatures killed in a short firefight can be reanimated within seconds to minutes by an infection form, and live creatures can be mutated into fully-grown and mentally devious combat forms: within minutes of contact, many Elites aboard the Infinite Succor were mutilated. At times, the mutations can occur randomly and un-uniformly: according to the Halo Graphic Novel, and can sometimes develop into engorged combat forms notably larger than the normal combat form or can alarmingly grow thick shells within minutes, indicating that the Flood can facilitate tremendous and exaggerated metabolic activity during the physiological mutilation. Furthermore, in rare cases, the physiological mutations can constantly shift: when Rtas 'Vadumee battled his comrade-turned-combat form Kusovai aboard the Infinite Succor, Kusovai continued to mutate throughout the duration of the battle, suggesting the possibility that over time any infected host can mutate further and further, not between Flood form classes, such as the Brain Form or the Gravemind, but into different permutations of combat form.

Trivia

 * There was never a Grunt or Jackal combat form seen. These are instantly turned into Carrier forms, as they are useless for combat in their current state. However, in the level "Sacred Icon" in Halo 2 there is a rare glitch when killing one of the carrier forms, a yellow armored grunt will emerge from it dead.
 * The first known Human to be infected by the Flood was a UNSC Marine named Private Riley.
 * In both Halo and Halo 2, Combat forms are always seen wielding two-handed weapons, such as the Shotgun, with one hand, and never seem to need to reload them. Just like other AI, the Flood never run out of ammo.
 * It could be said that the one responsible for releasing the Flood is 'Qualomee, because he told Jacob Keyes of the Covenant "weapons cache". However, it could be argued that the Flood escaped earlier, since the Marines found dead Covenant throughout the installation including an Elite with a hole in its torso. Though, if this were caused by an infection form, it seems odd that it did not remain in the Elite, turning it into a combat form. Kappus adds that the Covenant worked hard to seal the door where the Master Chief first encounters the Flood. Also, 343 Guilty Spark tells Master Chief that he believes the Covenant responsible for releasing the Flood.
 * In Halo 2, if you destroy both arms of a combat form, for some reason, they will walk in one direction very slowly then explode leaving the infection form there. The reason this happens is unknown, but it might be a process for the infection form to get out of a damaged body. Or it may be Bungie's way of saving processing power by not allowing the player to keep so many enemies on the map. Also it may be possible that Bungie didn't want the player to have a number of "Friendly Flood" following them.
 * In Halo Combat Evolved, once both arms fell off they were unable to fight. They would still chase after you, but would not have any way of harming you. However, due to a glitch, human flood forms would often smack the player with an arm that does not exist.
 * A glitch causes green Flood blood often to show up in orange Hunter blood in Halo Combat Evolved.
 * if you use active camo around the flood the combat forms cant see you however the infectious forms can.