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 and History
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. Meanwhile, the Flood began to develop a centralized, hive-like intelligence: the Gravemind form, which sought to assimilate all sentient life in the galaxy.

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.

Although the Flood were temporarily contained by the first firing of the Halos, they eventually rose again. Another Gravemind creature was spawned, directing the Flood's efforts, although initially, the Flood remained dormant, merely kept in dormant Forerunner research facilities, carefully wardened by the Forerunner Sentinel drones. However, in 2552, the Flood were unleashed and began to exponentially multiply. When the Covenant stumbled upon Installation 04, "Alpha Halo", the Covenant began massive excavation projects and exploration of Forerunner facilities, believed to have been attempting to recover Forerunner artifacts. In the ensuing Battle of Installation 04, the Covenant discovered the Flood, and interfered with the Forerunner defenses keeping them in check.

At a large underground Forerunner facility repurposed by the Covenant as an arms cache situated in a foggy swamp, the Covenant unlocked the Flood, which destroyed the Covenant garrison and a UNSC Marine expeditionary force, running rampant and reaching the ground, spreading on the surface. With a large source of vulnerable hosts in the form of the Covenant and the humans, the Flood began to exponentially reproduce again, and within hours, Forerunner security facilities and laboratories were overrun as the Flood, reinvigorated with a new source of hosts, overwhelmed the natural security measures and began to destroy the Sentinel custodians on Installation 04.

However, the most powerful of Forerunner containment measures had not been activated: the re-activation of the Halo rings, which would stop this Flood outbreak. Coordinating this was 343 Guilty Spark, the chief Forerunner intelligence on Installation 04, a "Monitor". However, the process was complex: according to Forerunner custom and security protocols, a human being, ritualistically termed the "Reclaimer", had to take an artifact, the "Index", from a Forerunner security facility, a "Library". The Reclaimer then had to enter the "Control Room", a massive chamber, and unify the Index with the "Core", a holographic control panel, and then the firing of the Halos would commence again.

343 Guilty Spark originally sought after a UNSC Marine, Staff Sergeant Marvin Mobuto, but he was overcome by the Flood. Then, the Monitor found another human to complete the ritualistic tasks: SPARTAN-117, a UNSC SPARTAN-II supersoldier, who tentatively consented, although SPARTAN-117 was mislead about how the Halo countermeasure would contain the Flood, and was not aware that it would extinguish all sentient life.

SPARTAN-117 was teleported to the Library, a Forerunner multitiered joint security facility and laboratory which contained the Index: the key necessary to prime the Halos for activation. While the uncontrolled massive propogation of the Flood caused numerous system failures in the Forerunner facilities, diverting 343 Guilty Spark's attention, SPARTAN-117 began to trek through the Library, and aided with a handful of remaining Sentinels, was able to make his way up to the top floor of the Library, where the Index was held, despite massive swarms of Flood resistance. Then, 343 Guilty Spark secured the critical Index and teleported SPARTAN-117 and himself to the Control Room, where SPARTAN-117 was given back the Index and was told to unify it with the Core. However, even as SPARTAN-117 did it, Cortana, a UNSC artificial intelligence of abnormal intellect, caused a systems error, forcing the Core to reject the Index. As SPARTAN-117 and 343 Guilty Spark were caught in surprise, Cortana informed SPARTAN-117 about the true workings of the Halo installations, that it contained the Flood by destroying all sentient life and depriving the Flood of hosts. SPARTAN-117 then made it his utmost priority to prevent the Halo installations from firing, denying the Monitor the Index.

At the time, Covenant forces, widely stretched across Installation 04, were massively engaged and being overwhelmed by Flood forces. While at places, the Covenant were able to hold the line by using vehicular and air support in the form of Ghost attack vehicles, Banshee fighters, and Wraith heavy tanks, Grunts and Jackals crumpled easily to the Flood, and the smaller number of Elites were being outnumbered and destroyed. The UNSC, having a much smaller presence, found out about the Flood on a widespread scale days later, when a UNSC Orbital Drop Shock Trooper force engaged with the Covenant nearby Alpha Base was ambushed by the Flood, with bitterly high casualties.

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.

Gravemind
The Gravemind form is the highest state of evolution that a Flood form can achieve. Once a "critical mass" of Flood forms have been syncytically fused, similar to the Brain Form and of a larger scale, a Gravemind form is created. It automatically assumes a centralized sentient intelligence that is believed to lead the general Flood species itself through unknown means, possibly telepathic. It also appears to hold all of the Flood's memories, even those of individual organisms, though extraordinary and enigmatic "genetic" means. While it is capable of directing the overall Flood species, it does not possess micro-managing capabilities for small encounters.

The Gravemind is the personification of Flood psychology, ravenously seeking to assimilate all sentient life in the galaxy, and is of unusual intelligence: of inventive plans for Flood propogation and serpentine delusions and betrayals, capable of manipulating others to do its bidding. It appears that only one is present at a time : one existed over 100,000 years ago, but was destroyed by the first firing of the Halos. Another was recreated afterwards, and laired in the depths of Installation 05: it showed remarkable understanding of Forerunner technology, even going asofar as to capture the resident Monitor, 2401 Penitent Tangent, manipulating the installation's teleportation system, and understanding the purpose and intracies of the Halo installations. The Gravemind is also remarkably long-lived amongst Flood forms, living for supposedly millenia, and has extremely keen sensory capabilities and understanding, "listening through rock and time".

Physically, it somewhat represents an engorged Venus flytrap: it is a "massive, tentacled...entity" with a gigantic, fleshy neck and indistinct head, prominently featuring a massive mouth that speaks in low, gravelly tones. It possesses massive tentacles of extraordinary length and strength, capable of stretching from the dark interior of Installation 05 and believed to be extending hundreds of kilometers to above the surface of the ringworld.

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

 * The first known human to be infected by the Flood was a UNSC Marine, Private Riley.
 * The Flood Juggernaut was a Flood form intended to be featured in Halo 2, and was found in the game's encoding, but was never featured, just like the Engineers in Halo: Combat Evolved.
 * Blowing off a Flood combat form's limbs in Halo: Combat Evolved would cause the form to trail the player, as it would want to attack, but would not possess the appendages to do so, a trick often referred to as "How To Make Flood Friends", "Harmless when Armless", or "How To Make Flood Pets", although occasionally, the Flood form would attack with invisible arms. This was disabled in Halo 2.