Flood

The Flood (otherwise known as the Parasite by the Covenant) is a species of virulent, savage, 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 Forerunner, and their origins and purpose of coming to the Milky Way galaxy are unknown, but are presumably to infect all sentient life in the galaxy.

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.

Prehistoric History
The Flood are believed to have arrived 100,000 years ago (approximately in the year 97,448 BC), when they were perceived 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 "Conservation Measure", researching the Flood, their adversaries, while constructing the Ark on Earth(?), protecting the "worthy" human race from destruction from the Flood. Some Flood specimens 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, 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 in the galaxy, would contain the Flood. After all other measures failed, the Forerunners activated the Halos, unleashing galactic destruction, destroying almost all sentient lifeforms. Without lifeforms to feed on, the Flood will eventually die out. However, the human race, on Earth, was believed to be shielded by the firing of the Halos by the Ark construct. It should be noted that the Forerunners may also have built other Arks on many other planets to save races with a high potential to evolve high intelligence, such as the Sangheili, the Unggoy etc.

The Flood were contained, lacking hosts and thus unable to reproduce. The only surviving specimens were contained in top-class, high security Forerunner research facilities, such as the Halo installations and the Threshold Gas Mines. However, the Flood was released of their prison and managed to infect the Heretics and Covenant and destroy the Sentinel guards in the Gas Mine.

Battle of Installation 04
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 guarded 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, running rampant and reaching the ground, spreading on the surface. A UNSC Marine Corps expeditionary force of two squads, led by Captain Keyes, and including Staff Sergeant Avery J. Johnson, and Privates Kappus, Wallace A. Jenkins, Bisenti, Manuel Mendoza, and Riley, were sent to investigate the weapons depot, and were unaware it had been overrun by the Flood. The small Marine force was overwhelmed in seconds, and all were subsequently infested, although one of Sergeant Johnson's medical conditions would render him immune to Flood infection, and Jenkins would be incompletely infested, as mentioned below.

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 facilities on Installation 04 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, branching out to infest the surface.

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 to be the Reclaimer, 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 super soldier, 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 multi tiered joint security facility and laboratory which contained the Index: the key necessary to prime the Halos for activation. While the uncontrolled massive propagation 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 itself 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 stunned 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. However, one of the attacking combat forms a UNSC Marine, Wallace A. Jenkins, one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by an elderly infection form whose powers of infestation had been dulled by years of hibernation, making Jenkins's conversion into a combat form incomplete. Jenkins still held mental resistance against the pervasive Flood mind-being in his mind, and although incapable of articulate speech and with a physiologically devastated body, Jenkins was able to betray the Flood through inarticulate hand gestures during an interrogation with Major Silva, informing the UNSC Marine companies that the Flood planned to attack the UNSC base from underground. Jenkins was later captured for a source of intelligence against the Flood threat.

Although uncertain of intelligence provided by a "half-naked, killing zombie" whose allegiance and intelligence were doubtable, Major Silva, acting commander of all UNSC forces on the ring, directed a small detachment of ODSTs to secure the tunnels underneath the base, when they found to their great surprise the Flood were trying to burn through. At the minimal casualty of one soldier, the ODSTs were able to weld shut all apertures, denying the Flood the possibility to attack Alpha Base from underneath.

Meanwhile, SPARTAN-117, under Cortana's directives, set out to destroy three pulse generators in a particular set of snowy canyons near the Control Room, which would significantly damper Installation 04's damage if it was fired. By the time, Flood dominance over the Covenant was being assured. The Sentinels were extremely rare, and unable to contain the Flood. Meanwhile, whole rooms formerly held by the Covenant were already completely taken by the Flood, although Covenant and Flood forces continued to wage war on massive ice plains, with the Covenant with the benefit of Shade stationary plasma turrets and vehicular/aerial support and the Flood sporting M19 Rocket Launchers and massive numbers.

The Flood, however, were not unintelligent. One of the UNSC personnel captured in the Marine force to the ex-Covenant arms cache was Captain Keyes, a brilliant naval strategist of the UNSC Navy. When he was infested, instead of using him in combat, the infesting infection form, while deleting his memories, felt that Keyes held vital memories...Keyes knew the location of Earth, the birthplace and major planet of the human race, a planet with tremendous possible assimilation opportunities for the Flood. Instead of becoming a combat form, he was syncytically merged with at least two other human hosts into a massive, engorged Brain Form aboard the CCS-class Battlecruiser Truth and Reconciliation, a Covenant warship that the Flood were relentlessly attacking. While in the brain form state, the Flood began to comb his memories, but Captain Keyes found that he could "feed" the Flood the same information over and over by drawing it from his neural implants and so hide more important information, and heroically sacrificed himself to the Flood by ordering SPARTAN-117 to take the neural implants in his skull, thus ending his life and allowing Cortana to gain access to the Pillar of Autumm's engine cores.

The Truth and Reconciliation was a major point of contestation between the Covenant and the Flood. The Flood appeared to have entered the warship previously, decimating most of the crew and garrison, leaving isolated pockets of resistance, mostly of small numbers of terrified Grunts and a few Elites aboard the vessel. However, below the Truth and Reconciliation, in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Elites augmented by Grunts, Jackals, and Hunters, fighting off against the Flood. The Covenant high command on Installation 04, however, were terrified by the Flood presence. Because the CCS-class battlecruiser had been damaged in a previous space battle with the Halcyon-class Cruiser UNSC Pillar of Autumn, it had been grounded, awaiting repairs for at least two days. After the Flood were discovered to be aboard the battlecruiser, the Covenant High command sent a special operations strike force to the Truth and Reconciliation to commence immediate emergency repairs, prepare it for liftoff into space and neutralize the Flood. These efforts, however, failed, with the Flood eventually retaking the vessel. However, the UNSC eventually took an interest of the Truth and Reconciliation: after Cortana reported that SPARTAN-117 was going to detonate the UNSC Pillar of Autumn's fusion reactors to create a thermonuclear explosion that would destroy Installation 04, Major Silva decided that the other UNSC personnel on the installation had to escape...and an excellent choice would be the Truth and Reconciliation. If the surviving technical crew could effect emergency repairs to the battlecruiser while highly-trained "Helljumper" special forces operators cleansed the vessel of the Flood, the UNSC could possibly liftoff the alien vessel and bring it to Earth, and along with it, the first captured Covenant warship and a wealth of knowledge.

In a swift assault, the UNSC was able to achieve what the Covenant could not. In a lightning strike, UNSC-captured Banshee fliers converged on the ship, and ODST commando teams quickly secured critical points on the warship while naval personnel were ferried onboard and began to make repairs on essential systems that were critical for the liftoff. After establishing a series of defended points, the ODSTs began to expand to cleanse the rest of the warship from the Flood, with minimal casualties, and the Flood were believed to have been completely destroyed. However, there was tremendous risk: if a Flood-infected vessel traveled to Earth, if there was even "one" Flood form aboard, "Earth would fall", according to UNSC AI Wellsley, because of the Flood's exponential parasitic potential. The ambitious UNSC commander, Major Silva, refused to heed the advice, but his second-in-command, First Lieutenant McKay, was horrified by the possibility. However, in the end, it was the captured ex-Marine combat form Jenkins that turned McKay's conscience, and the imprisoned combat form urged her to destroy the vessel for the sake of humanity. During liftoff, McKay destroyed a fiber-optic conduit controlling the engines, crashing the vessel and destroying the Truth and Reconciliation, at the cost of hundreds of UNSC Marines, ODSTs, and naval personnel, but saving Earth from the Flood.

The Flood appeared to be interested in seizing space-capable vessels in the hope of spreading the Flood away from the Halo rings and into the stars: they also attacked the UNSC Pillar of Autumn. After the UNSC cruiser's crash-landing onto the ring after engaging over fifteen enemy warships, it had been seized by the Covenant, although a UNSC raid had managed to temporarily retake the hull. On the third and final day of the Battle of Installation 04, however, the Covenant were in control of the crashed warship, when a Covenant patrol was ambushed by the Flood, infested, and when it returned into the vessel, the Covenant garrison was attacked by Elites-turned-combat forms. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the Pillar of Autumn through rents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force onboard. Eventually, the Sentinels came onboard, overwhelming the dwindling Covenant forces as the Flood began to come into the warship in full force, in the end almost completely obliterating the Covenant presence and severely limiting the Sentinels onboard. Despite their best efforts and enormous numbers, the Flood were unable to stop SPARTAN-117 from detonating the wrecked warship's reactors and escaping while the thermonuclear detonation severed the ringworld, destroying its capability to annihilate all sentient life, and killing all remaining Flood on Installation 04.

However, this did not mark the end of the Flood, who were still widely disseminated across the galaxy.

Threshold Gas Mine
The Flood were also kept in the Forerunner-constructed Threshold Gas Mine, in one of the three peripheral arms of the station. A small Covenant special forces strike team of three Phantoms laden with Elite Special Operations Elites and Special Operations Grunts, led by Special Operations Commander Rtas 'Vadumee and the Arbiter, were dispatched by the High Prophets to kill Sesa 'Refumee, a Heretic Elite stationed within the mine that was encouraging dissent against the Prophets.

During the Arbiter's reckless pursuit of 'Refumee, the Arbiter, Rtas 'Vadumee, two Special Operations Elites and two Special Operations Grunts entered into a laboratory, and the Heretics deliberately locked the doors: most likely by design, in order to crush the Covenant Special Operations force, the Heretics deliberately released the Flood from containment in that arm of the station, locking the Arbiter and his five allies into a peripheral laboratory and then besieging them with the Flood while Heretics dueled the Flood underneath them. When that trap failed to annihilate them, 'Vadumee retreated to call for reinforcements and coordinate a continued assault while the Arbiter and any remaining allies pursued 'Refumee to a gigantic elevator that ran in the spine of the Flood-infested arm: while presumably used to transport gas conduits, its walls were rampant with ravenous Flood, and it ferried canisters full of infection forms. The tenuous, agonizing elevator was nicknamed "A Descent Into Hell" as the Arbiter's allies were slowly picked off as Flood attacked the elevator, with Sentinels continually hovering overhead and firing lasers into the stochastic fray. Finally, the Arbiter left the elevator into a second wide laboratory hall, where the Heretics had cleverly laid another trap, locking the doors while Flood forms poured in. The Heretics plan was not flawless, however: a handful of Heretic Elites were still present in the second laboratory when the Flood entered, and those Elites were crushed while a horrid tide of infection forms, combat forms, and carrier forms lurched into the hall as the Arbiter was forced to defend against them until the tide stilled and the doors were opened.

'Refumee's plan of unleashing the Flood upon the pursuing Covenant Special Operations strike force, however, was not nearly a precise plan. The Flood would not be contained into rooms and merely attack the Covenant strike force. Their voraciousness was too much for either the Heretics or their attendant Sentinels to contain, and the Flood passed on a pathway between their infested peripheral arm to the central core of the station, where they overwhelmed the Heretics there. The Arbiter was forced to pursue 'Refumee, but was heavily impended by large waves of Flood crushing outnumbered Heretic Elites, Heretic Grunts, and ever-dwindling number of Sentinels. Eventually, the Sentinel presence was completely obliterated as the Flood spread to the upper levels of the station. In a desperate attempt to prevent 'Refumee from hiding under shields of battling Heretics vs. Flood, the Arbiter critically damaged the station by unbalancing it, destabilizing it into a massive atmospheric storm that was coming. The gas mine would soon be destroyed in the storm, forcing 'Refumee's hand to escape. The sudden destabilization of the station did not strengthen the Heretic defenders, and the Flood pressed them back in all fronts, even controlling whole stretches of darkly-lit corridor in the chaos. Finally, the Arbiter killed 'Refumee, and escaped in a Phantom as the station was sucked into the maelstrom and all Flood and remaining Heretics perished.

Battle of Installation 05
Unlike on Installation 04, where the Flood had been contained by the Sentinels until the Covenant began to meddle with the Forerunner containment measures, the Flood had been in continual war after the firing of the Halos 100,000 years ago: even up to 2552, the Flood were locked in a stalemate against the Sentinels. However, like on Installation 04, the fresh suitable host bodies of a newly-arrived Covenant armada and a UNSC strike force fueled the Flood's exponential parasitistic capabilities, and Flood infestation increased at a surprisingly fast rate: within hours, Sentinel forces were being overrun.

In order for the activation of the Halos and the facilitation of the ultimate Flood containment measure to occur, a Reclaimer had to retrieve the Index from Installation 05's own Library, making the Library a high-priority target for the Flood: it if could control the Library and prevent any Reclaimer from entering, it would make itself essentially invulnerable. Installation 05's Library was nestled in a tenuous series of wintry and snowy canyons, and boasted three levels of defense: an exterior Sentinel Wall kilometers in the distance, an underground honeycombed facility that created large amounts of Sentinels that impeded any invaders from advancing, a Containment Shield, an energy shield that denied access by any means to the Library until it was deactivated, and a Quarantine Zone, a biological "airlock" that would prevent the Flood from spreading any further. This multitiered defense was presided over by hundreds of Sentinels and Sentinel Majors, and a handful of Enforcers, massive, shielded aerial vehicles that were extremely effective against vehicles and infantry alike.

However, two external forces, the Covenant and the United Nations Space Command, wanted the Index as well: the Covenant, to activate the Halos and set off the "sublimation event" of the Great Journey, and the UNSC, to secure the Index and prevent the Covenant from doing so. The Covenant sent an elite strike team of Special Operations Elites, commanded by Special Operations Commander Rtas 'Vadumee and led into battle by the Arbiter, to retrieve the Index in a multi-staged assault. However, first, the Covenant needed to deactivate the Containment Shield by disconnecting its power generator in the center of the Sentinel Wall. Previously, a force of Grunts and Jackals had been dispatched to complete that operation, but they were overwhelmed by Sentinel defenders and forced into defending positions instead of attacking ones. The Arbiter was personally ordered by the Prophet of Truth to find and retrieve the "Sacred Icon" (the Covenant term for the "Index") for the Covenant. Because the Containment Shield was activated by the Sentinels, he could not be directly air-dropped to the Library. Instead, he had to land at the periphery of the Sentinel Wall, disable the generator, and then proceed. Tartarus deployed the Arbiter in a Phantom dropship, where the Arbiter demounted, entered the first half of the Sentinel Wall, where there was evidence of heavy Covenant casualties and abandoned infrastructure, making his way down through the facility and finally deactivating the Containment Shield. While the first half of the Sentinel Wall had remained pristine, the second half, the one bustressing the Quarantine Zone, was infested by Flood. Sentinel resistance had been destroyed in some segments of the Flood-Infested Wall, and in heavily infested areas, the Flood appeared to be beginning to terraform the Forerunner facility, spreading murky spores into the air. The UNSC deployed a number of Marines to fight through the Flood-Infested Wall, but they were grossly overrun. The Arbiter was able to battle through hundreds of Flood forms that had taken residence in the Flood-Infested Wall, which had entered through wall-mounted conduits, although later, the Covenant deployed a team of three Special Operations Elites in the lower levels of the Flood-Infested Wall, although those were killed as well. Finally, the Arbiter was able to make his way out of the Flood-Infested Wall, emerging at the ground levels: the Quarantine Zone.

With the Sentinel Wall compromised by the Covenant and the Flood, and the Containment Shield lowered by the Arbiter, there was only one last line of Forerunner defenses: the Quarantine Zone. While there was a minimal Sentinel presence there, there was a significantly elevated number of Enforcers present: there were even Forerunner constructors build into the cliff walls that automatically constructed Enforcers, further swelling their numbers even in intense combat. Here, the Flood demonstrated their unusual technical aptitude, commandeering Covenant and even UNSC vehicles, possessing over two dozen Ghosts, Wraiths, Warthogs, and Scorpions to do battle with the hovering Enforcers, although the Flood also boasted a large number of Combat Forms as well, which appeared to infiltrate the Containment Zone by means of arced chasms that periodically punctured the ground, emitting a green mist and appearing to be of indefinite depth, perhaps linkages to the Flood-infested underground.

Six Special Operations Elites, along with Rtas 'Vadumee, were airdropped at the periphery of the Quarantine Zone by means of Covenant drop pods, and the Arbiter joined them, rallying their efforts to secure an LZ against large numbers of Flood attackers. Using the advantages of elevated ground, Shielded Plasma Cannons, and a Deployable Lookout Tower, the small Special Operations contingent was able to hold an LZ for a Phantom to drop off a Spectre and a pair of Ghosts. Meanwhile, 'Vadumee commanded his men to assist the Arbiter in forging towards the center of the Flood-infested Quarantine Zone, so that the Arbiter may reach the Library and take the Sacred Icon.

The Flood, despite a significant vehicular and infantry presence in Quarantine Zone surrounding the Library, they were unable to deny access to the Covenant and the UNSC. The UNSC managed to slip a Pelican Dropship through the Forerunner Enforcer patrols, while the Covenant launched a groundborne attack through the battling Flood and Sentinels led by the Arbiter.

Attack on High Charity
SPARTAN-117, who had arrived with Keyes and Johnson, and the Arbiter met for the first time in the clutches of The Gravemind, the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. The Gravemind attempted to convince The Arbiter against his own beliefs, through the use of the captured Monitor, 2401 Penitent Tangent, and the semi-revived Prophet of Regret. Still unswayed, although in question of his religion, the Arbiter prompted The Gravemind to show him, by "stopping the key from turning." Transporting SPARTAN-117 and The Arbiter to High Charity and Delta Halo's Control Room respectively.

Taking advantage of the distraction that SPARTAN-117's presence caused, as well as the ensuing Covenant Civil War, the Gravemind took the opportunity to deliver himself as well as a multitude of Flood underlings onto In Amber Clad and crash into the citadel. A wave of Flood controlled Pelican Drop-ships delivered Flood all over the city. The ill-equipped and distracted populace where no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the Prophet of Mercy falling victim to the infection.

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems; the whole city was rapidly being terraformed into a massive Flood hive. The words of comfort the Prophet of Truth delivered to to the city's population, were regularly rebutted by The Gravemind over the comm system, to sadistically instill more panic and unease. SPARTAN-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the Forerunner Ship that was leading the Covenant fleet to Earth as well as transporting the Prophet of Truth. Cortana, who had to stay behind to manage the situation should Installation 05 be fired, was left in the clutches of the Gravemind, from which he could learn all Human Knowledge.

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 and simple Flood form, and is the smallest, although it is believed to be the most essential: 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 separate 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 augmented 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 Infection 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 volatile 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.

Juggernaut Form
The Flood Juggernaut is very large and is approximately fourteen feet high. The Juggernaut's tentacles can kill any living force in its path in one enormous blow. The Juggernaut is even capable of destroying Sentinel Enforcers. With its freakishly long legs and grasping arms, it would have no problem grappling an Enforcer out of the sky and ripping it apart. The Flood Juggernaut can leap twice as far as the already ridiculously far jumping Combat Form.

It is not known what infected creatures form Juggernauts. They seem to have ribs located in their midsection's, which means they cannot be Hunters, since Hunters are made up of orange worm-like tentacles that resemble a single conscious. It is possible that they are in fact infected Brutes, but this is less-widely believed. Another theory is that the Juggernauts are a mixture of Flood Carrier Forms or Combat Forms.

Flood Juggernauts have various features synonymous with carrier forms, such as a large, bloated cephalic area, and two long tentacles. However, Juggernauts seem to be better suited for attack like combat forms. This could be an indication that Juggernauts are a mutation of either carriers or combat forms.

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 infection: 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 synthetically 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, through 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 propagation 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 so far 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 fly trap: 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 Halo.

Infection


The Flood species parasitic existence is fueled only by its infection and assimilation of other species, which is done by the Flood's infection forms. The infection forms latch themselves onto a potential host and infects them until the host has become a Flood Combat Form.

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 infected 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 impediments, 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 prodigious arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. Metabolic 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 incapable 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 conflagration of highly volatile gas.

Countermeasures
Because of the millennia 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. Avery J. Johnson, having Boren's Syndrome, escaped the containment facility in which two squads of UNSC Marines were attacked by the Flood.

Combat Behavior
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 infecting 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 clumsy 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 code, 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.