Flood

The Flood, (Latin Inferi redivivus, meaning "the dead reincarnated" ) also known as the Parasite by the Covenant, is a species of parasitic organisms that thrive by consuming all sentient lifeforms of sufficient biomass and cognitive capability. They present the most variable faction in the game. This includes Humans and most Covenant species. The Flood also absorbed the Forerunners during the Forerunner-Flood War

The exact origin of the Flood is unknown, but it is known that they are an extra galactic species. They are naturally inclined to attack and invade the bodies of any intelligent being with a central nervous system that they encounter, and so their ultimate goal is consuming all sentient life throughout the entire universe. There are five stages of development that the Flood goes through: Pre-combat, Feral, Coordinated, Interstellar, and, if the infection spirals too far out of control, Intergalactic. In Pre-combat stage the flood spores will infect any biological mass nearby and start growing mycelia on it very much like it does to an infected body. This will go on until the mass is large enough to form an Infection form, In the Feral stage, the Flood makes use of the four basic forms: Infection, Combat, Carrier and a Proto-Gravemind Form (Brain Form). Once a viable Gravemind has developed, the Coordinated stage begins and the numbers of forms grows rapidly as more beings are infected. The Flood reach the Interstellar stage once they have acquired sufficient technology from the absorbed races and no further evolutionary changes occur. The Intergalactic stage is the last known stage for the Flood, and it happens once the organism has consumed all sentient life in a galaxy (and by extension, gained complete understanding of any technological and scientific knowledge that was present to it). It will have reached a critical mass and so must depart to other galaxies, possibly by either entering a dormant state or by utilizing said gained knowledge and technology for further replication. In retrospect, the Flood follows a systematic flow of replication similar to the lysogenic and lytic cycles of viruses, but on a universal scale as opposed to a cellular level. The Flood's potential for destruction is not to be under estimated. It is an organism-scale parasitic species, and its sole means of reproduction is by assimilating other sentient species into its fold by neurological and physiological mutagenic means, by entering and infesting live bodies or recent corpses. Its all-consuming nature and dependence on sentient life means that it would be impossible for the Flood to be a part of any ecosystem. They adapt quickly to any ecosystem and use its properties to its advantage.

Forerunner-Flood War
The Flood are believed to have arrived to the galaxy more than 150,000 years prior to 2552 in 98,379 BCE, when they encountered the Forerunners on the planet G617g, although it is unknown where the Flood actually came from, planet-wise. The Flood used unarmed civilian assets to penetrate planetary defense groups, first striking at the Forerunner-held planet of LP 656-38 e, bypassing the Forerunner orbital fleet and infesting the planet below, soon moving to similarly infest DM-3-1123 b. They were perceived as hostile by the Forerunners, whose ground forces were overcome by the sheer numbers of Flood forms on the ground. While at first the Flood demonstrated a lack of even "basic cohesion", they had "numerical superiority"; billions of forms to dedicate for the assault and every member of the Forerunner population a potential new Flood form. In desperation, the Forerunner's armada was ordered to immediately commence full planetary bombardment on infested worlds, although at great cost- many Forerunners were unable to be evacuated before bombardment commenced, leaving the Forerunner military initially with Pyrrhic (desperate, costly) victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers appeared to activate localized weapons of mass destruction, destroying themselves before the Flood could infest them in "noble sacrifice[s]".

Thereafter, the Forerunners realized that ordinary naval tactics were unable to stem the growing tide of the Flood, and began to invest in super weaponry. While the Forerunners initially deployed the Sentinel robotic drones to fight against and contain the Flood, almost immediately afterwards, the Forerunner Fleet Command contemplated "premature stellar collapse": using naval battle groups to destroy planetary star systems' primary stars in supernova that would engulf the planetary systems and prevent any possibility of Flood infection.

While the Flood were exponentially growing, spreading from system to system with the intent to infest the Forerunner populations, not to wage war with Forerunner battle groups, the tide was turning: The Flood had reached sufficient proportions to spawn a Gravemind form, a creature that embodied a collective sentient intelligence for the Flood species, and one that could coordinate the Flood swarms in active attacks against the Forerunner fleets. The Forerunners were aware of this, and constructed the Contender-classification artificial intelligence Mendicant Bias in an effort to destroy this centralized Gravemind and disorganize the Flood.

The Forerunners began a "Conservation Measure", researching the Flood, while constructing the artifact that would create a portal to the Ark on Earth, protecting the human race from destruction from the Flood. Some Flood specimens were kept by the Forerunners for scientific study. At this time, a Forerunner called the Librarian was busy cataloging and storing every form of sentient life on board the Ark.

The Forerunners employed many, many measures, including intricate naval tactics, 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 life forms in the galaxy, would starve the Flood out. After all other measures failed, the Forerunners activated the Halos in 97,448 B.C.E., unleashing galactic destruction, destroying almost all sentient life forms. However, the human race and many other species on Earth and throughout the galaxy were shielded from the firing of the Halos by the Ark.

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

Release
Although the Flood were contained by the first firing of the Halo Array, samples of parasitic specimens were, kept alive on various Forerunner outposts for scrutiny. One such research site was located on Installation 04. This proved to be a grave mistake, as both Humanity and The Covenant stumbled upon the abandoned ring. The AI construct aboard the Human vessel Pillar of Autumn made a presumably 'blind' jump through slipspace (although it is later revealed that she used translated co-ordinates from a Forerunner artifact found on Sigma Octanus) to evade Covenant pursuers. This lead both species to Installation 04, (Note: evidence also shows that there were covenant cargo ships on Halo 1, not transports, it is reasoned that this shows they were looking for Halo's control room before the humans arrived) and in turn, the Flood. After the crash of Pillar of Autumn, the surviving crew dug in for what they believed would be a long and brutal campaign against Covenant forces on the ring. Meanwhile, their pursuers, the Covenant cruiser Truth and Reconciliation, amassed its forces on the surface and soon discovered an underground Forerunner facility in the swamps of Halo. Due to the nature of the facility, the Covenant came to use it as a fortified base of operations, and for housing weaponry; oblivious as to its true nature. When Captain Jacob Keyes is captured by the Covenant for interrogation, he catches wind of this weapons cache and, upon his rescue, organizes fire team Charlie to seize it. When the team arrives at the swamp however, they find the facility all but deserted. The few Covenant soldiers left are scattered, and put up little resistance. As Keyes delves further into the facility, he comes across more and more mangled Covenant corpses. The Covenant occupants of the research facility had continued to move through the structure in hopes of finding Forerunner artifacts, but instead accidentally released the living samples of the Flood. Knowing full well what they are capable of, the bewildered Covenant forces promptly locked the facility down and evacuated those who survived. Keyes and his team are unaware of this, and proceed deeper into the facility until coming across one of the Flood Containment chambers. Believing that the 'Weapons cache' awaits, they unlock the door and are promptly attacked and assimilated by the Flood. With a large supply of sentient life now occupying the ring, the Flood had a viable food source and was able to multiply and infest most of Halo. Soon the infestation proved too great even for Halo's automated systems to handle, and the acting construct, 343 Guilty Spark was forced to call upon a "Reclaimer" to activate the array once more.

Assault
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 a short while later, when a UNSC Orbital Drop Shock Trooper force engaged with the Covenant, near 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 infection had been dulled by years of hibernation, making Jenkins' 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 battalion that the Flood planned to attack the UNSC base from underground. Jenkins was captured for a source of intelligence against the Flood threat, and provided intelligence that the Flood planned to assault the UNSC base from a series of underground catacombs underneath the base, and the UNSC moved to secure them, thus preventing the Flood from staging another attack.

By that time, Flood dominance over the Covenant was assured. The Sentinels were unable to contain the Flood. Meanwhile, whole areas 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 having 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 infected, instead of using him in combat, the infesting infection form, while deleting his memories, felt that Keyes held vital information. 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 merged with at least two other victims 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, Captain Keyes successfully withheld information about Earth by constantly accessing information available on his Command Neural Interface such as his name, rank, and serial number, until the Chief arrived. However, his arrival was too late, for Keyes was already dead. It was at that time the Master Chief punched a hole through Keyes' skull to get the Captain's CNI.

The Truth and Reconciliation was a major point of contesting between the Covenant special operations units 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 battle cruiser 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 battle cruiser, 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. Later, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held Truth and Reconciliation, and managed to neutralize most Flood forms on the cruiser in a swift assault, and prepared the battle cruiser for take-off with emergency repairs to escape from the ring to Earth. However, there was tremendous risk: if the Flood-infected vessel traveled to Earth, if there was even "one" Flood form aboard, "Earth could fall," according to UNSC AI Wellsley (this has been later supported in Halo 3 when Rtas' Vadum lectures Lord Hood: "One flood spore can destroy an entire species!" ), because of the Flood's exponential parasitic potential. Against orders, Marine officer McKay destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel but saving Earth from the Flood.

Defeat
The Flood appeared to be interested in seizing space-capable vessels, in the hope of spreading itself away from the Halo rings and into the stars: they attacked the UNSC Pillar of Autumn, also. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships, it had been taken 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 vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. Eventually, the Sentinels came on board, 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 on board. Despite their best efforts and enormous numbers, The Flood,Covenant, and Sentinels were unable to stop SPARTAN-117 from detonating the wrecked warship's reactors and escaping while the thermonuclear detonation severed the ring world, destroying it and its capability to annihilate all sentient life, and killing all remaining Flood on Installation 04 plus any life that was on the ring.

This did not mark the end of the Flood..

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. While the Covenant Special Operations force attempted to track down and assassinate him, Refumee and his forces, by accident or design, unleashed The Flood that were held within the station.

During the Arbiter's pursuit of 'Refumee, in which the Arbiter neutralized several weapons emplacements, and enemy Banshees, 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 then 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 Infection Forms while Heretics dueled the Flood underneath them. When that trap failed to annihilate them, instead dealing tremendous damage to the would-be trappers, '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". 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 attempted to lock out both the Arbiter and the Flood. 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 annihilated while a horrid tide of Infection Forms, Combat Forms, and Carrier Forms lurched into the hall as the Arbiter was forced to defend himself against them until the tide stilled and the doors were opened.

The Flood would not be contained in rooms, and attacked both the Covenant assassination force and the Heretics themselves. 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 impeded 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. However, the Arbiter destroyed the entire Gas Mine, destabilizing it. The station was sucked into a maelstrom in the lower atmospheric depths of Threshold, very likely crushing all Flood and Heretics still onboard.

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 presumably dwindling with no fresh supply of hosts (perhaps involved more in hiding than open warfare). 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 parasitic capabilities, and Flood infestation increased at a surprisingly rapid 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: if it 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 multi-tiered defense was presided over by hundreds of Sentinels and Sentinel Majors, and a handful of Enforcers.

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 Brutes, 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 drop ship, 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 buttressing 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. There was a significantly elevated number of Enforcers present, even automated sentinel construction factories flying over watch, and automated factories built into the landscape, each churning out Enforcers and Sentinels, further swelling their numbers even in intense combat. Here, The Flood demonstrated their unusual technical aptitude, commandeering Covenant and even UNSC vehicles, possessing dozens of Ghosts, Wraiths, Warthogs, and Scorpions to do battle with the hovering Enforcers. Flood forces also boasted large numbers of Combat Forms, 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 a landing zone 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, were unable to deny access to the Covenant and the UNSC. Miranda Keyes managed to slip the In Amber Clad through the Forerunner Enforcer patrols, while the Covenant launched a ground borne attack through the battling Flood and Sentinels led by the Arbiter.

Attack On High Charity
SPARTAN-117, who had arrived with Keyes and Johnson, met the Arbiter 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. 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 to High Charity and the arbiter to 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, Gravemind took the opportunity to deliver himself as well as a multitude of Flood underlings onto In Amber Clad and then jumped the UNSC ship inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled Pelican Dropships delivered Flood all over the city. The ill-equipped and distracted populace were no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the Prophet of Mercy falling victim to an infection form.

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems; the whole city becoming Terra-formed into a massive Flood Hive. The words of comfort the Prophet of Truth delivered to the city's population were regularly rebutted by 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 Gravemind, who could potentially learn all possible Human knowledge from her.

Earth, The Ark, And The Flood's Defeat
''"Child of my enemy, why have you come? I offer no forgiveness, for fathers' sins passed to his son."

- Gravemind on the level Cortana  "I am a timeless chorus. Join your voice with mine, and sing victory everlasting."

- Gravemind on the level Floodgate '' At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Portal, and the Brute fleet escaping to the Ark, an infested Covenant cruiser slipped out of Slipspace and crashed into the city of Voi. In order to prevent the Flood from escaping and infecting the entire planet, Miranda Keyes ordered John-117 to detonate the cruiser's reactors and search the ship for Cortana, causing the city of Voi to be destroyed, and the Flood there eliminated. As John-117 and the Arbiter fought their way to the ship, the Shadow of Intent arrived and deployed an Elite task force to aid UNSC marines. Master Chief, with the help of the Elites, was able to battle through Flood forces, including newly encountered Pure Form Flood to reach the crashed cruiser where he found a message from Cortana. Upon this discovery, 343 Guilty Spark arrived pledging his assistance to the Reclaimers. With the Flood spreading, Master Chief boarded a Phantom while the Elites went on to glass the infected areas. The forces, fulfilling their mission of removing the Flood from Earth, retreated back to the Elite carrier, the Shadow of Intent, along with Cortana's message. With the assistance of 343 Guilty Spark, Cortana's message was repaired, revealing a solution to the Flood menace on the Ark. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with the Arbiter's and Rtas 'Vadum's Elites through the Portal.

On the Ark, while attempting to lower the shield surrounding the Ark's citadel, the infested High Charity made a slipspace jump and crashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built replacement for the destroyed Installation 04. After retrieving Cortana from the derelict High Charity, SPARTAN-117 was able to kill all Flood present on the Ark and the replacement Installation 04 by activating the unfinished Halo using the Index Cortana retained from the first Halo. The ring's activation also presumably killed the Gravemind. The Master Chief also destroyed High Charity by blasting its reactors causing them to go critical and killing the billions of Flood that were present when High Charity was destroyed in a huge explosion to destroy the flood.

Aftermath
With Gravemind destroyed, and all the Flood on the Ark gone, it is more probable that the Flood still exist on the other Halo rings, excluding Installation 05 (although it is possible that all of the Flood on Installation 05 didn't leave and there are still some there as that Installation was never destroyed) or any other remaining Forerunner Installations. The fact that the Flood were never originally from this galaxy to begin with further complicates the issue. Perhaps that is why the Forerunners preserved some Flood for research on the Halo rings instead of destroying them all, since the Flood would return sooner or later and it would be better to fight a researched enemy rather than a foreign one. For now, the Flood are seemingly defeated. Also, after the activation of the replacement Installation 04 in Halo 3, Gravemind sent one final telepathic message to the Master Chief, claiming that this was "another long stretch of time," for it, suggesting that it may still exist, despite the destruction of High Charity and the Ark.

Developmental Stages
The Flood go through 4 distinct developmental changes.
 * 1. Feral stage is the lowest form communicating via pheromones, and have the natural instinct to harvest enough calcium to establish a viable Gravemind. (Seen in Halo 1 and 2.)
 * 2. Co-ordinated stage is when the Flood become truly dangerous; they now take orders from the Gravemind that was created in the first stage. (Seen in Halo 2)
 * 3. Interstellar stage allows the Flood to take control of most technology and spread throughout galaxy to infect more hosts. (Seen in Halo 3)
 * 4. Transgalactic the last known stage the Flood enters, theoretically, in which it utilizes all captured technology to depart to uninfected galaxies to further replicate. (Not seen).

Biology


The Flood, a parasitic species, infects individual organisms and causes them to undergo different stages of organized transformation into various "forms". A Flood's chosen form depends on the species of creature infested, and the usefulness of the infected body. All Flood biomass is made up of the differentiable Flood Super Cell.

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

Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater (in early stages of life), the Flood can withstand harsh environments of all known colonized planets.

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 primal urge 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 indistinct: 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. They can use the vocal cords of Brutes, Humans, and Elites to communicate as well as the moans they use as observed on the level "The Covenant", where the drop pod crashes into the third shield tower. If you hide behind one of the spires you can sometimes hear them say things like "Where is he"(Uncommon), "Don't hide from me"(Common), and "You will be ours"(Rarely). It is likely that this is Gravemind speaking through the infected hosts, as he does this just before you team up with him towards the end of "The Covenant." Gravemind also speaks through Tank Forms. However, it is unknown how he does this since they are pure flood matter, unless the Flood engineered Tank Forms to have vocal chords.

Physiological
While baseline neurological assimilation is the 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 are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hit box-selective incapacitation impossible; organ functions are decentralized, thus, 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 claw like 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 blood-colored vessels spawning all across the creature's surface, most prominently so on the head.

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

There is no confirmed way to effectively stop a Flood infection when a host is infected by an Infection Form. However, a possible exception 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, it has been suggested that 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. However, there is some reason to suspect that this was due to his past in the ORION Project, and that the Boren's Syndrome explanation was invented to hide it. The truth in this matter remains uncertain.

While Boren's Syndrome left untreated may leave you immune to the Flood, leaving it untreated usually equates to death within a couple years. There is said to be a one in a billion chance that research from someone so inflicted with Boren's Syndrome in a way that they cannot be infected could be researched on to produce a cure.

Since the Flood are a parasitic life form, they depend on other organisms for survival. Thus, the only logical foolproof way to eliminate them is to take away their food sources. The Forerunners attempted this by bombing large population centers, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The absolute last resort was the use of the Halo installations, which effectively destroyed all life in the galaxy and starved the Flood.

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 infect organisms of other species, physically adhering to their surfaces and inserting tentacles, forcibly "hacking" into its host's nerve system, completely changing its psychology to that of the Flood's, and eventually physiologically mutating it into one of the various Flood forms.

Aquatic Form
The Proto Form is never encountered in combat and its use by the Flood is not certain. They were on the level Oracle inside a small glass and metal tank. Since the player cannot get to them physically, they cannot die and do not react to anything. Whether they are a natural Flood form or a Forerunner experiment is not clear. The only indication that they are alive is that they float in a school in circles around the tank. They could be "Baby Infection Forms" still developing as they would have in the sacs found in flood hives. There is also a chance that they are used to infect aquatic creatures. Another possibility is that a young version of a particular type of form's growth had been stopped by the liquid in the tank. The Proto Forms are suspiciously similar in appearance to the Flood Drop Pods of Halo 3; it has been theorized that they are in fact immature versions of the drop pods. They can also be mistaken as Horseshoe Crabs which are probably what Aquatic Forms were based on.

Infection Form


The Infection Form is the most basic and simple Flood form, and is the smallest, albeit the easiest to defeat, although it is believed to be the most essential: it is the Flood's method of infecting and assimilating other species as these vile creatures attack in dozens or even hundreds. Their discovery on Halo indicated that either they can remain dormant for countless millennia until a suitable host awakens them or they were kept alive for research purposes. 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. As these changes happen, the infection form will deflate its gas bag, pull itself into the host body, and spread its tentacles throughout the body. However, energy shielding provides some protecting against an Infection Form, popping the creature in a small, but damaging, blast of noxious gas.If you pop one thats in a pack it will cause a chain reaction.

After infecting viable hosts, 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 a horrible assimilation within minutes. But in Halo 3, the coordinated-stage infection forms transform their victims into Combat Forms very rapidly in just a few seconds. Infection forms can't infect beings with strong armor, such as Master Chief, or Brute Chieftains. 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. The Infection form is most easily defeated with the M6D pistol, as the explosive rounds can take out multiple Flood at once.

The only known human survivor of a Flood infection attack was Sergeant Johnson, because of Borens Syndrome, and Master Chief. Johnson's potential status as a Spartan-I means that his survival may have been due to his augmentations, rather than the Borens Syndrome cover-story.

Combat Form


After an Infection Form has successfully developed into a sentient creature with sufficient biomass and 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 infection. Infection forms have been seen only mutating humans, Elites and Brutes, not bothering to utilize Grunts or Jackals presumably because of their physical weakness and lack of significant biomass and combat capability, instead turning lower creatures into Carrier Forms and, possibly, because of the fact that it would take a longer amount of time to make a Carrier form and the growing infection forms inside.

Combat forms are more dangerous at close range, utilizing their whip-like mutated arms to lash out with scourging blows of significant force, 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 an Elite, Brute, Marine, or Spartan.

As mentioned below, combat forms can use any weapon that their host body might once have used, also utilize weaponry or vehicles, although they are poorly proficient with almost all firearms and are clumsy with vehicular operation. They have been seen to utilize active camouflage as an Elite form. Combat forms are rakishly aggressive and lack even rudimentary battlefield tactics, although their sheer numbers can force even powerful enemies to succumb. Infected Elites, Brutes or Humans can be transformed into combat forms within seconds,and corpses of previously infected hosts can be alarmingly reanimated in seconds by other infection forms, making corpse destruction a useful tactic when combating the Flood, and is even recognized by the Forerunner Sentinels, who sometimes incinerate infected corpses to ensure that they can not be reanimated.

Carrier Form


The exact origin of Carrier Forms is unknown, although they are theorized to be combat forms that are too injured, damaged, or aged to be combative for further use, or smaller and weaker life-forms (like Grunts and Jackals) who have been infected but lack the calcium mass to serve as combat forms, or both. The purpose of a Carrier Form is to nurture and disseminate Infection forms upon attack or proximity to a host..

Physically, they are a large, bloated sac situated upon a torso and a pair of waddling legs that slowly transport Infection Forms. Their bloated "sac"-like form is a result of the many Infection forms crawling around inside them. In combat situations, once they are within close proximity of a potential host or when their sacs are perforated even with a single projectile, forcing the carrier form to its pudgy knees while the sac exponentially swells with gas for several seconds, then spontaneously detonates into fleshy scraps, hurling infection forms into the air. The detonation of itself is extremely damaging, and its gas-propelled dissemination of infection forms facilitates easier assimilation, as it hurls infection forms in all directions, and stuns the potential hosts with the concussive blast. This explosion also has the potential to set off any stray grenades, making it even more deadly.

In comparison to Halo: Combat Evolved and Halo 2, Halo 3's Carrier Form is much more difficult to destroy, and it releases  more Infection Forms. However if set on fire or is hit by a plasma or spike grenade, no infection forms would be released, making flame and plasma excellent weapons against such forms. The explosion caused by a needler is just as effective.

Pure Form
A new stronger strain of Flood that develops once a coordinated stage has secured sufficient resources namely in the form of calcium from host species. The Flood make use of those reservoirs of calcium to develop an adaptable framework upon which the Flood Super Cell was attached which creates a fearsome biological war machine. Initially, this form's primary function was to aid Infection Forms by securing more biomass in the shape of hosts as well as enough complex neurosystems to strengthen the local Gravemind. This duty overlaps with the Combat Form but once enough Pure Forms have developed, the Combat Forms are relegated to defensive roles or adding to the biomass and calcium reservoirs.

They can transform into another pure form within seconds; a stalker form usually turns into a tank form when you get close to it, and a ranged form if you're trying to kill it and there's a wall nearby; ranged forms mainly turn back into stalkers to transform into tank forms, and tank forms may be mistaken for a Hunter form, mainly because a tank form has one oversized hand (equivalent to a Hunter's shield), a small arm (equivalent to where it's gun once was), it's roar which sounds similar to that of a hunter, and even more similar body mass.

The ranged form is susceptible to the Covenant Needler. It also is VERY useful to use plasma, spike, and firebomb grenades on a ranged form. The ranged form can also fold itself well remaining on the wall; when folded, it is much harder to kill. They can sometimes be heard talking in Halo 3. It is apparently the Gravemind speaking through them, as all Flood in an area speak with an echoing near simultaneity, in the same tone, speaking the same sentence. They use taunts such as "I will be your doom", and calling the player's armor a "casket".

Prophet Form


Prophet Forms are rare products of infestation and assimilation. Because of this, 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.

Brain Form


One of the rarest types of Flood is the Brain Form (a.k.a Proto-Gravemind). This form is unlike all other Flood forms since it lacks any sort of mobility, defense, and is passive. These are the beginning stages of a Gravemind form. It is believed to be the product of syntactical fusion of several combat forms, merging all of its subordinate forms into a massive, bloated spherical organism with spindle-like outgrowths that adhere to the surrounding environment.

Proto-Gravemind 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 and mental communication with other humans. 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.

The only known human example currently is Captain Jacob Keyes, who was taken by the flood in combat form originally, and was then merged with the Proto-Gravemind on the bridge of the CCS-class battle cruiser, the Truth and Reconciliation.

Gravemind


The Gravemind is the leading intelligence of the Flood. There can only be one at a time, although if one is killed another may form. The Gravemind is the assumed highest state of transformation that a Flood form can achieve. It is believed that once a "critical mass" of Flood forms have been synthetically fused, similar to the Brain Form, but on a larger scale (and very likely the next stage of a brain form), a Gravemind form is created. It 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, and of the previous Gravemind as well, through extraordinary and enigmatic "genetic" means. While it is capable of directing the overall Flood species, it also 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 intricacies of the Halo installations. The Gravemind is also remarkably long-lived, surviving for millennia.

Physically, it somewhat resembles 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.

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, often times 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 preference for close-range combat: combat forms possess abnormally strong melee attacks by whipping their tentacles, capable of dropping a SPARTAN-II 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 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 could 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. The Flood were also gathering body parts for a new Brain Form 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 micro-jump for the frigate and perform rudimentary maneuvering 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.

The Flood's millennia 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 mutated. At times, the mutations can occur randomly and uniformly: according to the Halo Graphic Novel, they 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. Though never seen, it can be assumed infection forms were involved in infecting victims in this case. Before being killed to prevent his becoming a Flood, one of the Elites accompanying Rtas 'Vadumee appears to have a wound in his chest that may have been the entry point for an infection form. Many of the Flood on the Succor seemed to constantly be mutating the infected biomasses and also seemed to merge them together. This suggests the possibility that over time any infected host can mutate further and further, as exemplified by the Brain Form or the Gravemind and different permutations of the Combat Form as exemplified in the Carrier Form being a further mutated Combat Form.

Tactics
HALO 1: It is best to use the Assault Rifle, Shotgun, or Pistol against the Flood. Don't bother meleeing them, it takes five melees to kill one combat form. It is a good idea to melee the infection forms though, especially if you are in combat. That prevents them from draining your shield too much. IF there are lots of infection forms, then you shouldn't worry about it, shooting one will cause a small blow affect which will send a chain reaction through any other infection forms standing around it. They will then blow too.

HALO 2: Dual-SMGs and other dual-wield weapons have proven to be very useful, as well as the Sentinel Beam, Shotgun and Energy Sword. However, Plasma Pistols will be the less useful. Dual-wielding Plasma Rifles and aiming at the arms of the Flood is more than enough to rip them apart easily. These will come off quickly and therefore leaving the combat form literately armless. They will woddle about and explode after a few seconds. The SMG is also as good against the Flood as Halo 2, as long as ammo is in large supply. Also effective is the Battle Rifle, as aiming for the Infection Form in the chest of the Combat Forms will kill the Infection Form controlling the Combat Form in a single burst. It is often a good idea to use the brute shot melee or energy sword on "dead" flood (as well as living flood)as it will destroy the body and prevent infection forms from bringing it back to life.

HALO 3: Sticking Carrier Forms with plasma grenades or using a flame-based weapon against them will kill them and their cargo of Infection Forms. Energy Swords are the best weapons to use against Pure Forms, as even on Legendary, they will still kill a pure Tank Form with one hit, but are closely followed by the Brute Shot, as the melee attack kills and incapacitates all forms of Flood in one hit, except for Tank Forms, which take two hits. The fact that you can use the melee as many times as you want makes the Brute Shot more useful than the Energy Sword in situations where there are extremely large numbers of Flood. Like in Halo 2, it is also a good practice to melee dead elite and brute combat forms in order to prevent infection forms from bringing them back to life. Gravity Hammers are also good for large groups of Flood, but aren't nearly as effective against pure forms as Energy Swords. The shock wave from the hammer can burst the pods containing Infection Forms, resulting in more enemies. If you manage to get a Flamethrower, they are very helpful against especially pure forms, as it will almost always kill them instantly even on Legendary difficulty. The Assault Rifle is excellent for Combat Forms and infection forms alike, though it is unfortunately ineffective against Pure Forms

Using a Battle Rifle, if you shoot the heart of most combat forms, this will act as a head shot and subsequently kill them instantly.

Perhaps the surprisingly most effective way to take down the Flood from a distance is the ubiquitous Plasma Rifle. On all difficulties a few searing shots will be efficient enough to take down all forms; perhaps the best overall ranged weapon.

Halo 2 (from Halo: CE)

 * The Flood can make screeches and roars which were not present in the first game.
 * Skin is more textured, giving the Flood a more realistic and disgusting look. Faces are also more recognizable on humans, a touch many players find scary.
 * If a Combat Form has both arms shot off, it will walk slowly then explode, releasing an Infection Form. This eliminated the "Flood Pet" glitch.
 * Rather than having to "gather bodies," the Flood can seemingly infect hosts over the course of a few minutes.
 * Rather than corpses re-animating themselves, an Infection Form will enter the body, then re-animate it.
 * flood forms can now blow up
 * If a player, Grunt, Jackal, Elite, or Brute is jumped by an Infection Form while their shields are down, they will die instantly. In Halo: CE, it would simply take a bar of health away.
 * Some flood Elites still have their energy shields functional.
 * The Gravemind is introduced, making the Flood a more significant faction in the series.
 * The Flood can use vehicles, albeit clumsily.
 * The melee attack does little to no damage against the Flood in Halo: CE. It does a bit more damage in Halo 2.
 * The Flood appear to be "smarter" when in combat, preferring to stay back and fire on the player rather then "rush" him/her.
 * In Halo: CE a Combat Form would walk slowly when shooting at the player. When losing the arm that shoots, it would go "berserk": charging at the player, madly flailing its arms. In Halo 2 the combat forms can "decide" whether to use a melee attack or to shoot at the player.
 * If a player is wielding an Energy Sword or Shotgun, a Combat Form may retreat. They will not do this if a player is wielding other weapons.

Halo 3 (from Halo 2)

 * An infected Brute combat form was added.
 * Infection Forms can mutate humans, unshielded Elites and Brutes into ready for action Combat Forms in around seven seconds (another seven reference).
 * Flood Pure Forms are introduced, consisting of the Tank Form, the Stalker Form, and the Ranged Form.
 * Flood Growth Pods are introduced, growing on walls in ships and buildings.
 * Flood Drop Pods are introduced.
 * Gravemind can speak through his minions during combat.
 * Combat Forms wield weapons far less than in both Halo: CE and Halo 2.
 * Melee attacks were useless in Halo: CE and Halo 2, but can be used effectively against the Flood with any weapon.
 * Carrier Forms release significantly more Infection Forms.
 * Carrier Forms are a lot harder to kill.
 * The Shotgun is significantly more effective against them.
 * If you are hit by an Infection Form while your shield is down it will not only make a crunching noise, but it will most likely kill you.
 * Most Flood (Mainly Combat Forms) are weaker and more vulnerable to melee attacks from regular weapons.

Trivia

 * When the I Would Have Been Your Daddy skull is on in Halo 3, two things will happen
 * 1) In the background noise, murmurs can be heard from Combat Forms that sound like, "Braaiinnssss..." This is a reference to the fact that Flood are often considered "Zombies"
 * 2) Their noises sometimes sound like: "I see you" if you are hiding in a corner.
 * The first known Human to be infected by the Flood was a UNSC Marine named 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, much like the Engineers in Halo: Combat Evolved.
 * The Flood can infect nearby corpses in Halo 3. If you leave a weapon near the corpse about to be infected, the newly infected Combat Form will pick it up and use it, depending on what weapon it is.
 * 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 A Flood Buddy", although occasionally, the Flood form would attack with invisible arms. This was disabled in Halo 2 and Halo 3. This instead results in the death of the Flood prior to the loss of the last appendage. When the infection form comes away from the body
 * Part of the Flood's Binomial nomenclature, Inferi, is Latin for "The Dead", and could be a reference to Roman mythology, as this was their name for the Netherworld and it's inhabitants. Inferi means 'The Dead' and Redivivus means 'renovated, reconstructed, rebuilt or recycled' so literally Inferi redivivus means 'The Dead Reconstructed'.
 * Combat and Pure Forms seem to have the ability to "talk" in Halo 3, although they only use this ability to serve as mouthpieces for Gravemind.
 * In Halo 2 and Halo 3, Combat Forms may suffer a head shot if hit in the chest, quickly destroying the Infection Form in the chest cavity. This makes it easy to take them down using Battle Rifles or Pistols, but not overly useful unless there aren't any Infection Forms around to reanimate those downed in that way.
 * There is a species of fungi that is frighteningly similar to the Flood, the cordyceps unilateralis. They attack carpenter ants and take over their neural system in order to make the ants go to high places. By the time it gets there, its been rotted from the inside out like a Flood host and it releases spores from that vantage point.
 * In Halo: Combat Evolved, it is possible to melee a Combat Form to death, although it will take a very long time before it dies. In Halo 3, one melee will kill almost all Combat and Pure Forms, including the Brute and excluding Tank Forms.
 * In Halo Combat Evolved, driving over a Carrier Form with a Warthog will not cause much damage to the player. However, in Halo 3, driving over a Carrier Form with a Warthog will flip the Hog over because the Carrier Form's explosion will have the force of a grenade.
 * In Halo Combat Evolved, in the level 343 Guilty Spark, it is possible to see a few combat Forms before going into the structure in the beginning of the level. You need a Pistol to see them, though. They can be seen on the large tree stump that is far away from you.
 * In a PC World article, the Flood were voted as #31 of the top 47 "most diabolical (evil) video game villains of all time."
 * In Halo 3, you can stick a corpse just before it is re-animated. Once animated the sticky grenade seems to have disappeared (This works with all sticky grenades including the Incendiary Grenade).(This also happens when you stick a "pure" form flood in mid-transformation)
 * It is possible to infect Sgt. Johnson into a Combat Form in the last level of Halo 3.(Kill him before you walk into the control room)
 * In Halo 3 Flood blood splattered looks like symbols from Bungie's last game Marathon.
 * In Halo 3, occasionally, you might come across Spec Ops Elite Flood Combat Forms that can take a lot of damage. Such as being able to withstand 3 Energy Sword lunges or 4 Gravity Hammer hits. But encounters with these forms are rare.
 * In Halo 3, If the player has their back towards an Infection Form, occasionally it will grab the players head and drain their shields.
 * An infection form's tentacles can be seen from a combat form's mouth.
 * Even in large numbers, Infection Forms can be taken out with only a couple shots from the Assault Rifle because the Human shots ricochet off into another Infection Form taking out numerous Infections at once.
 * After time passes, a Combat Form will transform into a Carrier Form.
 * If an Elite combat form is hosed with the assault rifle, being downed and NOT destroyed, it will expose the host's skull. When the infection form revives the flood, the skull will move the mandibles of the fallen Elite.
 * In Halo CE, it was advised not to fight Flood with melee attacks, but by Halo 3, it was a preferred close-combat attack.
 * If a Flood Combat Form is neutralized but the Infection Form in it's chest is uninjured the infection form will remove itself from the original host to find a new one (seen only in Halo 3).
 * In both Halo 2 and Halo 3, once a combat form's corpse is incapacitated (i.e, the infection form inside the cavity is shot) the lifeless body with give out a scream or growl.
 * In Halo 3, the human combat forms all have the same uniform, so if a marine with specialist uniform gets infected, the uniform it will change to the normal ones.
 * In Halo 3 there is a color glitch with the elite combat forms. when you blow up there bodies the armor is always blue (red and grey turn to blue when blown up).
 * The Sniper Rifle System 99C-S2 AM is ineffective against the flood due to their insides being liquified.