Flood: Difference between revisions

{{Status|Canon}}

*"Shaping Sickness" ([[Ancient humanity|ancient human]]){{Ref/Novel|Pri|Page=53}}

The '''Flood''', designated as '''[[LF.Xx.3273 Flood Super Cell|LF.Xx.3273]]''' by the [[Forerunner]]s ([[Latin]] '''''Inferi redivivus'''''<ref name="bestiarum">''[[Bestiarum]]''</ref> meaning "the dead reincarnated"<ref>[http://carnage.bungie.org/haloforum/halo.forum.pl?read=865888 ''Halo.Bungie.Org'': Updated and abbreviated list, errors amended]</ref>) and referred to as '''the Parasite'''{{Ref/Level|H2|[[The Heretic]]|Quote=Thel 'Vadamee: ''"Noble Hierarchs, surely you understand that once the parasite attacked-"''}} and '''the infection'''{{Ref/Level|HW|[[Relic Approach]]|Quote=[[Re'gish Wamik]]: ''"What if the infection defiles the [[Relic (location)|relic]]?"''}} by the [[Covenant]], is a species of highly virulent parasitic organisms that reproduce and grow by consuming [[sentience|sentient]] lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the galaxy - including the vast majority of [[Forerunner]]s - during the [[Forerunner-Flood war]] in ancient past, prompting the [[Great Purification|activation]] of the galaxy-sterilizing [[Halo Array]] in [[97,445 BCE]].{{Ref/Level|CE|[[Two Betrayals]]}}

Originating as a corrupted form of the galaxy's most ancient custodians (the [[Precursors]]){{Ref/Novel|Id=precursors|Sil|Page=173-175}} the Flood reproduces and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized forms. Through infecting sentient lifeforms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.  

The ensuing conflict ended with the Forerunners successfully wiping out all but a few Precursors.{{Ref/Novel|Pri|Page=335-336}} The few Precursors that escaped from (or were spared by) the Forerunners either went into suspended animation or transmuted themselves into dust that would regenerate into their past forms at a later time. However, over time the dust became defective, creating sickness, disease and biological mutations in other organisms that came into contact with it. With this new form — the earliest stage of the Flood — the Precursors vowed that none of their creations would rise against them again.{{Ref/Reuse|precursors}} A significantly mutated Precursor, one that would later be known as the last surviving one of its kind,{{Ref/Book|HM|Page=9}} was sealed in a Precursor [[stasis capsule]] and placed in a small, ravaged planetoid at the galaxy's edge around [[9,000,000 BCE]].{{Ref/Novel|Sil|Page=181-182}} This being, later known as the [[Primordial]], would eventually reveal the nature of the Flood to those willing to know the truth.{{Ref/Reuse|precursors}}

The Flood, as it is known in modern times, first appeared in the galaxy some time prior to [[107,445 BCE]], after [[Ancient humanity|ancient humans]] came into contact with the Precursor dust. This fine, desiccated powder had been stored in millions of glass cylinders placed aboard automated starships,{{Ref/Novel|Id=origin|Cry|Page=268-270}} bound toward the margins of the Milky Way galaxy from the direction of the [[Path Kethona|Large Magellanic Cloud]].{{Ref/Novel|Id=sil35|Sil|Page=35}} [[Forthencho]] came to believe that the [[Primordial]], despite its hibernating state at the time, may have been responsible for remotely triggering the movement of these ships.{{Ref/Novel|Pri|Page=226}} However, over a span of ten million years, the substance had slowly deteriorated, along with the ships themselves failing.{{Ref/Reuse|origin}}

The crashed remains of these ships were discovered by ancient humans on both inhabited and deserted worlds near the border of [[intergalactic space]], a region of the galaxy to which humans expanded early on in their history. Inside the ships, humanity found the capsules filled with powder, which their tests told them consisted of inert, short-chain organic molecules of unknown purpose.{{Ref/Reuse|origin}} Humanity studied the material and deemed it to be harmless, but noted that it had psychotropic effects on lower animals. Small dosages were [[Drugs|administered]] to [[Pheru]], popular domestic animals among humans and San'Shyuum, which resulted in more docile behavior.{{Ref/Novel|Id=cryptum131|Cry|Page=131}} Unbeknownst to the humans or San'Shyuum at the time, the powder was altering the genetic code of the Pheru, manifesting as improved behavior while slowly changing other aspects of their genome.{{Ref/Reuse|origin}}

The infection quickly spread through both human and Forerunner space. Attempts to quarantine infected worlds and systems failed, leading to the Flood soon infecting hundreds of worlds in fifteen systems.{{Ref/Reuse|origin}} In an effort to halt the Flood's continuing spread, human fleets began destroying Flood-infested Forerunner ships and cleansing Forerunner planets where Flood infestations were detected. [[Forthencho]], the commander of the human fleets, chose not to warn the Forerunners in advance of these actions to prevent giving the Flood time to expand.{{Ref/Game|Id=terminals4|H4|Detail=[[Terminal (Halo 4)|Terminals]]}} In turn, many Forerunners believed that the Flood was a fiction meant to hide humanity's expansionist aims. Ultimately, the Forerunners interpreted humanity's tactics as acts of aggression, leading to the [[Human-Forerunner wars]].{{Ref/Reuse|origin}}  

With humanity severely weakened from the Flood infection and now fighting a two-front war, the Forerunners were surprised at the ease of their earliest victories against the humans. However, apparently without warning, the Flood began to retreat from its assault on humans. Forerunner ships found healthy human worlds within sectors of Flood infestation, leading Forerunners to conclude that humanity had developed a natural resistance, immunity or cure to Flood infection.{{Ref/Novel|Id=sil34|Sil|Page=34-37}} However, no immunity or cure to the Flood existed; the Flood had chosen to stop infecting humans.{{Ref/Novel|Id=p364|Pri|Page=364-365}} The [[Ur-Didact|Didact]] later speculated that the Flood's intent was to prevent humanity's extinction at the hands of the Forerunners by making it appear as if humans held a solution to Flood infection.{{Ref/Novel|Sil|Page=170}}

At this time, the Forerunners also first came into contact with the Flood. The parasite devastated many of their fleets, before its deliberate recession from the galaxy tipped the scales in the Forerunners' favor, allowing them to sterilize any Flood infestation they came across in several thousands of worlds across the galactic border.{{Ref/Reuse|sil34}} While recognizing the legitimate concern of a Flood resurgence in the future, the Forerunners had difficulty extracting any intact specimens of the parasite for study; approaching the threat with appropriate caution, the [[Warrior-Servant]]s directly cauterized any active infestations they encountered. From the scattered remains recovered for analysis, the Forerunners discovered that AIs that had been in contact with the Flood had been corrupted beyond repair by what the Forerunners would come to know as the [[logic plague]], essentially the data equivalent to Flood infection.{{Ref/Reuse|sil34}} No fully intact specimen of the Flood could be found among the Pheru, humans, or San'Shyuum, as they had methodically destroyed any sign of infection, including all the original carrier vessels that they had discovered.{{Ref/Reuse|c272}} Any remaining Flood were believed to have fled the galaxy aboard infected ships,{{Ref/Novel|Id=c272|Cry|Page=272}} and the Flood was not seen again for nearly ten thousand years.

Due to the lack of evidence, most Forerunners were unaware of the existence or of the true nature of the Flood, and many dismissed it as a simple excuse for humanity's aggressive expansion before and during the initial arrival of the Flood. However, several high-ranking Forerunners were fully informed of the threat, and in secret began developing countermeasures should the Flood ever return. This became the source of a major political strife, with the [[Builder]]s proposing the construction of an array of superweapons - [[Halo Array|Halos]] - while the [[Warrior-Servant]]s led by the [[Ur-Didact|Didact]] championed a more conventional strategy of carrying out precision military campaigns from planet-sized fortresses known as [[shield world]]s. The Didact eventually lost to the Builders and the entire [[Warrior-Servant]] rate was marginalized, with their fleets and forces disbanded or forced to submit under Builder control. This proved to have been a grave mistake, as the Forerunners found themselves ill-prepared to fight off the Flood after the parasite returned to the galaxy.

A common tactic used by the Flood was to use huge numbers of unarmed civilian vessels to penetrate planetary defense groups, bypassing the Forerunners' orbital [[Forerunner fleet|fleet]]s, and infesting the planet and its populace below. The sheer numbers of Flood forms on the planets overwhelmed Forerunner ground forces. Billions of forms were dedicated to planetary assaults, and every member of the Forerunner population was a potential Flood host. These tactics were demonstrated in the Flood's attacks on numerous worlds, including [[LP 656-38 e]] and [[DM-3-1123b]].{{Ref/Game|Id=terminals|H3|Detail=[[Terminal (Halo 3)|Terminals]]}} In desperation, the Forerunner armada was ordered to commence full [[orbital bombardment|planetary bombardment]] on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before the bombardment commenced, leaving the Forerunner military with shallow victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers were seen to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.{{Ref/Reuse|terminals}}

Thereafter, the Forerunners realized that conventional naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on more extreme measures. While the Forerunners initially deployed robotic [[Sentinel]] drones to fight against and contain the Flood, almost immediately afterwards, the [[Forerunner Fleet Command]] contemplated "Premature stellar collapse": using naval battle groups to send stars into [[supernova]]e that would engulf their planetary systems and prevent any possibility of Flood infection and destroy all Flood present in the system.{{Ref/Reuse|terminals}} As a further setback to the Forerunners, an extended dialog with the Primordial provoked [[Mendicant Bias]], the [[Contender-class artificial intelligence|Contender-class AI]] in charge of the Forerunner defense, to succumb to the Flood's [[logic plague]] and turn its vast resources against its masters. While the Primordial's physical body was eventually destroyed, it's consciousness was either consumed or transferred to the Gravemind, granting the Flood previously unprecedented intelligence and power.{{Ref/Book|HM|Page=32}}

The Flood grew exponentially, spreading from system to system with the intent to infect the Forerunner population, not to wage war with Forerunner battle groups which were poorly adapted to such strategy. As the Flood spread further, entire planetary biospheres were converted to Flood biomass. These planet-sized [[key mind]]s, locuses of Gravemind intelligence that began to outcompete the Forerunners through raw computational might alone.{{Ref/Novel|Id=key mind|Sil|Page=186-187}}<ref name="deluge">[https://www.halowaypoint.com/en-us/news/deluge-delights ''Halo Waypoint'': "Canon Fodder - Deluge Delights"]</ref> The logic plague continued to take on more adaptive forms, defying all defenses the Forerunners devised and eventually spreading itself between AIs and data networks. As the Forerunners relied significantly on AI automation in combat, this severely hindered Forerunner military efforts against the Flood. Adding to these difficulties was the disturbing revelation that the Flood was capable of manipulating [[Precursor]] technology: towards the close of the war, the Flood began to appropriate Precursor megastructures such as the [[star road]]s, wielding the previously-inert strands to pummel Forerunner planets and naval forces. This ability was a result of the Flood's Precursor origins; being corrupted Precursors, they could tap into [[neural physics]] and activate Precursor artifacts. The proliferation of high-level key mind nodes also enabled the Flood to use the Precursors' neural physics-based superluminal transit, which in turn encumbered Forerunner slipspace travel. Towards the end of the infestation, Forerunners noted that space itself seemed to become hostile and unpleasant from mere perception, an artefact of Flood-based Neural Physics. {{Ref/Reuse|key mind}}

The Forerunners employed countless measures to contain the Flood outbreak, all of which failed. Finally, the Forerunners were forced to a Pyrrhic solution: using an array of seven ring-shaped megastructures collectively known as the [[Halo Array]] as superweapons, the Forerunners would purge the galaxy of all sentient life. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts (and in particular hosts capable of sentience) the Forerunners reasoned that eliminating all potential Flood hosts would render the parasite unable to grow and spread, and eventually cause it to starve to death. Additionally, as a result of the Master Builder's test of [[Installation 07|a Halo]] at [[Charum Hakkor]], it was found that the Halo Effect unintentionally caused the complete destruction of artefacts of Neural Physics. This was likely a byproduct of how the Halo Effect was designed to destroy any and all forms of neurological structure subjected to it.{{Ref/Novel|Id=cryptum 293|Cry|Page=293-294}} The decision led to political battles and even a civil war, but after exhausting every countermeasure, the Forerunners activated the Halos in [[97,445 BCE]].

As a result of the firing of the seven Halo installations, the surviving Flood were successfully contained and eventually died out. The only surviving specimens were contained in state-of-the-art, high-security Forerunner research facilities, such as the Halo installations, certain Shield Worlds, and the [[Threshold gas mine]]. All Precursor artifacts and structures, composed and maintained entirely using Neural Physics were also destroyed and left no trace.{{Ref/Novel|Sil|Page=326}} Many sentient species throughout the galaxy (including humanity) survived the firing of the Halo Array, having been safely relocated beyond the galaxy by the Forerunners on [[Installation 00|the Ark]].  

 

During their expansion throughout the Orion Arm, the [[Covenant]] would find and occasionally release the Flood, employing [[Zealot]]s and their [[Temple Wraith]]s to contain the outbreaks.{{Ref/Game|H5|List of REQ cards|REQ card|Detail=[[Temple Wraith]] description}}{{Ref/Book|Enc22|Page=224}}

Thousands of years after the end of the Forerunner-Flood war, the human ship {{UNSCShip|Spirit of Fire}} jumped from [[Arcadia]] to follow the signal of [[Ellen Anders|Professor Anders]], who had been captured by the Covenant. They were led to an unknown planet, which happened to be [[Trove|a Forerunner shield world]]. Containment measures on this planet had failed catastrophically, and the entire surface of the megastructure was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, [[John Forge|Sergeant Forge]] and several Marine squads were attacked by the Flood.

After successfully defeating the Flood forces, the UNSC forces created a [[firebase]] in the area that Anders' signal appeared to be coming from. The humans eventually tracked the signal to a massive [[Proto-Gravemind]] attached to several [[Flood colony|Flood colonies]]. The Proto-Gravemind had duplicated Anders' signal, possibly in hopes of infecting the humans and spreading beyond Etran Harborage. It was discovered that killing a Flood colony weakened the Proto-Gravemind and [[SPARTAN-II]] [[Red Team]], with help from the firebase, destroyed the Proto-Gravemind.

With the Proto-Gravemind dead, the smart-[[AI]] [[Serina]] was able to reacquire Anders' actual signal, which led to an ocean under which a docking port leading to the interior shield world was located. During the ''Spirit of Fire's'' descent, the outer surface of the ship was infested by the Flood. With the aid of Sentinels and "cleansing rings" controlled by the installation's AI, the ''Spirit of Fire'' crew were able to cleanse the ship and proceed into the shield world.

Within, the UNSC forces discovered that the Flood had managed to infest the interior of the shield world as well, though not to as massive of a degree as the surface planet itself. While rescuing Anders, Forge had to save her from three [[pod infector]]s and ran into some trouble with Flood forces on his following missions, though not to the massive extent they had been seen on the exterior planet.

In order to prevent the Covenant from getting their hands on Etran Harborage's fleet of Forerunner {{Class|Sojourner|dreadnought}}s, a plan was created to use the reactor from the ''Spirit of Fire's'' [[Shaw-Fujikawa Translight Engine]] to induce the shield world's artificial star to go [[supernova]]. The plan was a success and Etran Harborage was destroyed. The explosion obliterated the Flood on the planet, along with the Covenant forces and the Forerunner fleet.{{Ref/Game|HW}}

====Brief recurrence====

On January 1, [[2537]], six years after the events on [[Trove]], a Flood outbreak was initiated aboard the {{UNSCShip|Spirit of Fire}} by a [[Pod infector]] that had managed to remain hidden aboard the ship. As the Flood threatened to breach quarantine, Serina awoke [[Jerome-092]] who, with the aid of Professor [[Ellen Anders]], was forced to exterminate all infected crew members, ensuring the survival of the ''Spirit of Fire'' and its remaining crew. All traces of the Flood parasite aboard the ''Spirit of Fire'' were erased.{{Ref/Comic|TFS|Detail=''[[Something Has Happened]]''}}

{{Quote|The only way to stop the Flood is to starve them to death.|[[Cortana]] to [[John-117]] and [[343 Guilty Spark]].}}

Meanwhile, the pursuing Covenant cruisers, among them the ''[[Truth and Reconciliation]]'', amassed their ground forces on the surface, where they soon discovered an underground Forerunner [[Flood containment facility]] in the swamps of the ring. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and as a storage area for weapons and ammunition, oblivious to its true nature and purpose.

Keyes and his team were unaware of these events, and proceeded to move deep into the facility, ultimately coming across one of the unopened Flood containment chambers. Believing it was a weapons cache of some sort, the team unlocked the door, and were attacked and assimilated by the Pod infectors inside. Upon being informed of Keyes' disappearance, John-117 was deployed to Captain Keyes' last known location on the drop ship ''[[Echo 419]]'', proceeding into the facility, only to find out that most of the Marines he had been assigned to rescue, including the Captain himself, had succumbed to the Flood.  

These events brought knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.{{Ref/Game|Id=CE|CE}}

Shortly after their escape from the containment facility, Flood forces took control of a nearby crashed Spirit dropship, the ''[[Brilliant Gift]]'' and crashed it aboard the Covenant {{Pattern|Mjern|agricultural support ship}} ''[[Infinite Succor]]''. The Flood quickly spread throughout the ship, infecting its crew and the animals from the vessel's hunting preserve. The Flood began building a [[Proto-Gravemind]], intending to use the ''Infinite Succor'' to flee the [[Soell system]] and begin infecting other planets. The ship's Legate, the [[Minister of Etiology]], locked out the ''Infinite Succor's'' primary systems while one of the crew's [[Unggoy]] managed to send out a distress call. Suspecting human involvement, particularly that of [[John-117|the Demon]], [[Supreme Commander]] [[Thel 'Vadam]]ee sent a [[Covenant Special Operations]] team led by [[Rtas 'Vadum]]ee to retake the ship and eliminate the Demon if he were onboard. Quickly uncovering the true threat, Rtas programed the ship to make a [[slipspace]] jump into the system's star which would destroy the ''Infinite Succor'' and the Flood infestation onboard. Though Rtas succeeded, he was the only survivor.{{Ref/Comic|HGN|Detail=''[[The Last Voyage of the Infinite Succor]]''}}

After the Flood's escape, the Covenant forces across [[Installation 04]] were overwhelmed by the Flood's assault. While the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as [[Unggoy]] and [[Kig-Yar]] succumbed easily to the Flood, and the few Sangheili who remained were outnumbered and thus infected. The UNSC, having a much smaller presence on the installation, found out about the Flood's might a short while later, when an [[Orbital Drop Shock Troopers|ODST]] force engaged with the Covenant near [[Alpha Base (Installation 04)|Alpha Base]], was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, [[Private First Class|PFC]] [[Wallace A. Jenkins]], who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly Pod infector whose powers of infection had been dulled by centuries of hibernation, making Jenkins' transformation into a combat form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulate speech and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with [[Major (UNSC)|Major]] [[Antonio Silva]], informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their current base of operations. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.{{Ref/Reuse|TF}}

One of the UNSC personnel captured by the Flood was [[Captain (Navy)|Captain]] [[Jacob Keyes]], a brilliant naval strategist of the [[UNSC Navy]]. When he was infected, the Flood sensed that he held knowledge that would be especially useful to the Flood. Keyes knew the location of [[Earth]], the birthplace and major stronghold 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 four other victims into a massive, engorged proto-Gravemind aboard the {{Pattern|Ket|battlecruiser}} ''[[Truth and Reconciliation]]'', a Covenant warship the Flood had taken over. While he was being incorporated into the proto-Gravemind, the Flood intelligence began to dig through his mind, searching his memories for the location of Earth. 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 John-117 arrived. However, his arrival was too late, for Keyes was already dead, completely consumed by the [[Proto-Gravemind]]. It was at that time that John-117 was forced to punch a hole into Keyes' skull to obtain the Captain's CNI transponder.{{Ref/Level|CE|[[Keyes]]}}

The ''Truth and Reconciliation'' was a major point of contention between the [[Covenant Special Operations]] units and the Flood. The Flood had decimated most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Unggoy and a few [[Sangheili]] 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 Sangheili augmented by Unggoy, Kig-yar, and [[Mgalekgolo]], fighting off the Flood. The Covenant [[Council of Masters]] in charge of operations on Installation 04, however, were terrified by the Flood presence. Because the Covenant battlecruiser had been damaged in a previous space battle with the {{UNSCShip|Pillar of Autumn}}, it had been grounded, awaiting repairs to its underbelly and plasma carrying conduits, for at least two days. After the Flood were discovered to be aboard the battlecruiser, the Covenant military leadership sent a Special Operations strike force to the ''Truth and Reconciliation'' to commence immediate emergency repairs, prepare it for lift-off 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 battlecruiser for take-off with hastily completed emergency repairs to escape from the ring and return to Earth. However, there was tremendous risk if the Flood-infected vessel traveled to Earth, if there was even one Flood carrier form aboard, "''Earth could fall,''" according to UNSC [[Artificial intelligence|AI]] [[Wellsley]], because of the Flood's exponential parasitic potential. Against orders, Marine First Lieutenant [[Melissa McKay]] destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood.{{Ref/Novel|Id=TF|TF}}

Eventually, a force of [[Sentinel Aggressor|Sentinels]] came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels' numbers on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop [[John-117]] from detonating the wrecked warship's reactors, and escaping in a [[GA-TL1 Longsword|Longsword interceptor]]. The resulting thermonuclear detonation from the destabilized fusion reactor cores severed a portion of the ring world, ultimately destroying it and its capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were still present on the ring.{{Ref/Reuse|TF}}{{Ref/Reuse|CE}}

After the destruction of Installation 04, in October 2552, a human [[prison ship]], the ''[[Mona Lisa]]'', arrived in the Soell system and managed to retrieve surviving specimens of the Flood for study from the remains of Installation 04. They began to test the parasite on both human and Covenant prisoners, unaware of its lethality. Soon, the infection broke quarantine and the ship was infested by the Flood. The Flood absorbed the crew and prisoners, then began to build a proto-Gravemind in its engineering section. The infestation ended when Sergeant [[Zhao Heng Lopez]] and her marines from the {{UNSCShip|Red Horse}} were sent to investigate the ship after retrieving a [[John Doe|survivor]] from the ship. The Flood absorbed many of her marines, leaving her and a single [[Henry|Sangheili]] to fight for a single-occupant escape pod. The outbreak was most likely contained when the ''Red Horse'' destroyed the ''Mona Lisa'' using a [[Shiva-class nuclear missile]].{{Ref/Novel|Id=mona lisa|Evo|Detail=''[[The Mona Lisa]]''}}

The heretics' plan backfired as the Flood killed and infected many of their forces while failing to stop the Arbiter and his team. The Flood broke out of the station's arm and spread to the central core of the station, where they overwhelmed the heretics stationed there. Eventually, even the station's Sentinel forces were completely obliterated as the Flood spread to the upper levels of the station. The Flood were unable to escape the station, however, as the Arbiter managed to sever the cable attaching it to the Forerunner support structure far above, sending the gas mine plummeting to Threshold's liquid hydrogen core, destroying it.{{Ref/Level|H2|[[The Oracle]]}}

The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. [[Miranda Keyes]] managed to slip the ''[[UNSC In Amber Clad|In Amber Clad]]'' through the Forerunner Enforcer patrols, while the Covenant launched a ground attack, led by Thel 'Vadamee and involving a small team of Spec Ops Elites, using two [[anti-gravity gondola]]s to get to the index while attempting to control the gondolas and keep it from slipping into Flood or Sentinel hands. Later, the ''In Amber Clad'' was captured by the Flood.{{Ref/Game|Id=H2|H2}}

[[John-117]], who had arrived with Commander [[Miranda Keyes]] and Sergeant [[Avery Junior Johnson|Johnson]], met [[Thel 'Vadam]]ee 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 Thel, against his own beliefs, and through the use of the captured [[Monitor]], [[2401 Penitent Tangent]], and the semi-revived and consumed [[Prophet of Regret]] to not activate the ring. Even though Thel was still unswayed, although in question of his religion, the Gravemind sent him and John-117 to search for the Activation Index to keep the Covenant from "turning" the "key," transporting John-117 to [[High Charity]], and Thel to Delta Halo's [[Control Room]].{{Ref/Level|Id=Gravemind|H2|[[Gravemind (level)|Gravemind]]}}

Taking advantage of the distraction that John-117's presence caused, as well as the ensuing [[Great Schism]], the Gravemind took the opportunity to deliver itself, as well as a multitude of Flood underlings, onto the {{UNSCShip|In Amber Clad}} and then slipspace jumped the UNSC ship directly inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled [[D77-TC Pelican|Pelican dropships]] delivered Flood forms all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the [[Prophet of Mercy]] falling victim to a Pod infector.{{Ref/Reuse|Gravemind}}

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems within High Charity; the whole city was becoming terraformed into a gigantic Flood hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth regularly delivered to the city’s population over the comm system, to sadistically instill more panic and unease. John-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the [[Anodyne Spirit|Forerunner Dreadnought]] that was leading [[Truth's fleet]] to Earth. Cortana, who had to stay behind to detonate ''In Amber Clad's'' engines should the situation arise and Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible human knowledge from her, including the entire knowledge directly received from Forerunner systems aboard Installation 04.{{Ref/Level|H2|[[High Charity (level)|High Charity]]}}{{Ref/Level|H2|[[The Great Journey]]}}

[[File:Infection.jpg|230px|thumb|A UNSC Marine being infected by the Flood.]]

At the conclusion of the [[Battle of Voi]], with the [[Prophet of Truth]] activating [[the Portal]] and the Jiralhanae fleet escaping to [[Installation 00|the Ark]], a [[Indulgence of Conviction|Flood-infested Covenant cruiser]] appeared out of [[Slipspace]], having penetrated the Elites' blockade of High Charity and crash landed into the city of [[Voi]] in an effort to spread the parasite.{{Ref/Level|Id=The Storm|H3|[[The Storm]]}} In order to prevent the Flood from escaping and infecting the entire planet, Commander [[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 [[Thel 'Vadam]] fought their way to the ship, the [[Fleet of Retribution|Sangheili]] assault carrier ''[[Shadow of Intent]]'' under the command of [[Rtas 'Vadum]] arrived and deployed Sangheili task force to aid UNSC marines. John-117, with the help of the Sangheili, was able to battle through the Flood forces, including the newly encountered Flood [[Pure Form]]s to reach the crashed cruiser where he found a message from [[Cortana]]. Upon this discovery, [[343 Guilty Spark]] arrived, pledging his assistance to the [[Reclaimer]]s. With the Flood spreading, John-117 boarded a Phantom while the Sangheili went on to glass the infected areas. The remaining [[Sangheili]] forces, fulfilling their mission of removing the Flood from Earth, retreated back to the ''Shadow of Intent'', along with Cortana's message and Guilty Spark.{{Ref/Level|Id=Floodgate|H3|[[Floodgate]]}}

On the Ark, while the Sangheili and human forces were attempting to infiltrate Forerunner shield control centers and lower the shield surrounding the Ark's Citadel, the infested [[High Charity]] made a [[slipspace]] jump and smashed into the Ark, releasing the Flood onto the superstructure. The [[Gravemind]] then made a brief alliance with the Master Chief and the Arbiter, [[Thel 'Vadamee]], in order to stop the [[Prophet of Truth]] from activating the Halo Array, providing Flood backup to their [[Battle of the Citadel|assault]] on [[Installation 00 Citadel|the Citadel]]. Once Truth was dead, however, the Gravemind treacherously ordered the Flood to turn against the two heroes. The Chief and the Arbiter escaped, however, with some assistance from the Sentinels, who were able to help subdue the Flood with ease. As they escaped the Flood, the Chief and the Arbiter discovered [[Installation 08|a replacement Halo ring]] rising from the [[Foundry (location)|Foundry]] and realized that Cortana's plan to stop the Flood was to fire the new installation at the Ark, an act that would sterilize the local infestation while leaving the Milky Way galaxy unharmed.{{Ref/Level|Id=The Covenant|H3|[[The Covenant (level)|The Covenant]]}}

In order to fire the new Halo, the Master Chief [[Raid on High Charity|ventured into the derelict and wrecked High Charity]], now a massive Flood hive, where the AI was still held prisoner by the Gravemind. Facing increasing Flood forces and telepathic demands from the Gravemind, the Master Chief was able to rescue Cortana who still possessed [[Activation Index|the Index]] from [[Installation 04]]. In order to buy time to escape and to delay the Flood, the Master Chief detonated High Charity's reactors, destroying the hive and severely injuring the Gravemind while the Chief, Cortana and the Arbiter escaped on a Pelican.{{Ref/Level|Id=Cortana|H3|[[Cortana (level)|Cortana]]}}

The Chief took Cortana and the Arbiter to the new Halo ring, joined by [[Sergeant]] [[Avery Johnson]] in the {{UNSCShip|Forward Unto Dawn}} while the surviving humans and Sangheili retreated through the portal to Earth. However, the Gravemind survived High Charity's destruction and began to rebuild itself on the ring while the Flood [[Raid on Installation 08|invaded]], attempting to stop the new ring from being fired. Aided by Sentinels controlled by 343 Guilty Spark, the Chief, Johnson and the Arbiter were able to reach the [[Control Room]]. However, as firing the ring prematurely would destroy it and severely damage the Ark, Spark turned on the three, mortally wounding Johnson before the Chief apparently managed to destroy him. Cortana was able to use the Index from Installation 04 to fire Installation 08 which killed most of the Flood forms on the Ark along with the Gravemind. However, in its final moments, the Gravemind addressed the Master Chief and the Arbiter telepathically, warning them that their victory would only slow—not stop—the Flood. As predicted by Spark, the installation was destroyed by the firing and the Ark suffered heavy damage. The portal collapsed as the Chief and the Arbiter escaped on the ''Forward Unto Dawn'', leaving the Master Chief stranded in deep space.{{Ref/Level|Id=Halo|H3|[[Halo (Halo 3 level)|Halo]]}}

Even though the Gravemind was destroyed, and all Flood on the Ark apparently eradicated, the Flood still existed on the other Halo rings (excluding Installation 05 which was sterilized by Sangheili forces) and other remaining Forerunner installations. Even though [[Path Kethona]] was sterilized by [[Omega Halo]] near the end of the Flood war, given the Flood's extragalactic origin, it was possible that more pockets of Flood existed outside of the galaxy - or more disturbingly, that other, more distant galaxies may have fallen to the Flood.

Though the UNSC kept the Flood's existence concealed from the general population, it did make preparations for the possibility of the Flood's return. The new [[Spartan Operations|Spartan]] branch was designed as a first line of defense against the Flood, with the [[SPARTAN-IV program|SPARTAN-IV]] complement of the {{UNSCShip|Infinity}} running simulations depicting the possible return of the Flood. This is realized through both [[War Games]] [[Flood (gametype)|exercises]] and the interactive [[UNSC Tactical Simulator|TACSIM]].{{Ref/Game|Id=HSA|SA|Detail=[[Halo: Spartan Assault co-op missions|Flood missions]]}} Some Spartans volunteered to join [[Flood containment Fireteam]]s dedicated to going up against Flood outbreaks, with several outbreaks occurring following the end of the war and being successfully contained due to their actions.{{Ref/Reuse|OSFM}} Teams were deployed to quarantine any areas of potential Flood contamination during the UNSC's initial forays onto Halo [[Installation 03]],{{Ref/Book|H4EVG|Page=200}} though it is unclear whether the parasite was actually encountered. The UNSC has otherwise seen the development of the [[Hazops Cyclops]] dedicated to fighting the Flood threat, and the "Flood" variant of the [[Visual Intelligence System, Reconnaissance|VISR]] - given to Spartan personnel involved in Flood containment duties.{{Ref/Game|H5|Detail=Flood visor description}} The UNSC includes a briefing on the Flood in the [[Halo: Official Spartan Field Manual|Spartan Field Manual]], wherein it details protocols such as [[Emergency Contact Protocol Upsilon]] and the [[CORRUPTER]] cryptonym - to be declared in the suspected presence of a Flood outbreak and upon infection of a Spartan, respectively.{{Ref/Reuse|OSFM}}

Three days after the [[Battle of Installation 00]], with some Flood spores having survived the [[Raid on Installation 08|firing]] of [[Installation 08]] and the destruction of High Charity's reactor, [[Installation 00|the Ark's]] [[monitor]] [[000 Tragic Solitude]] had the ruined Covenant capital city of [[High Charity]] sealed off with a [[containment shield]] and the Ark's [[Sentinel]]s raze the perimeter and modify the refugia nearby so that no sentient life would survive in the vicinity, thus depriving the Flood of new host bodies were they ever to escape. Tragic Solitude also set a battalion of Sentinels on patrol around the perimeter, not only to keep the Flood contained, but to defend against any future outsiders who might try to enter the wreckage and thus unleash the Flood.{{Ref/Game|HW2|Detail=''Phoenix Logs'' - ''Idle Hands II''}}

{{Quote|I told you NOT to go inside. FOR GOOD REASON!|[[Atriox]] to [[Voridus]]{{Ref/Level|Id=Manifestation|[[Halo Wars 2: Awakening the Nightmare]]|[[Manifestation]]}}}}

[[File:HW2 FloodTankForm Concept.jpg|thumb|250px|A [[Flood tank form]] overwhelms a Warthog.]]

In early [[2559#June|June 2559]], after the loss of the ''[[Enduring Conviction]]'', the [[Jiralhanae]] brothers [[Pavium]] and [[Voridus]] were tasked to salvage the wrecks around [[High Charity]] for weapons to use in [[Second Ark Conflict|the conflict]] with the humans of the {{UNSCShip|Spirit of Fire}}. The [[Banished]] leader Atriox, aware of the potential danger of the Flood having survived in the city, gave specific orders to only scout the shell and not to go inside lest surviving Flood be disturbed. However, [[Voridus]], like many of the Banished, did not take the threat of the Flood seriously, believing it simply be another lie of the [[San'Shyuum|Prophets]]. Ignoring the warnings of both Atriox and his own brother Pavium, Voridus deactivated the Ark's Sentinel defense network and used a [[Barukaza Workshop Scarab]] to burn a hole in the [[containment shield]] around the former Covenant holy city. When Voridus' troops entered the wreckage to salvage it, they were ambushed by thousands of Flood [[infection form]]s lurking within High Charity which were then set loose upon the Ark. With the Sentinel defense network down, the Flood were left unhindered by the Ark's defense systems.{{Ref/Level|Id=What Could Go Wrong?|[[Halo Wars 2: Awakening the Nightmare]]|[[What Could Go Wrong?]]}}

Pavium quickly assembled a force to rescue his brother, defending against overwhelming Flood forces before finally retreating to meet up with Voridus at a Banished drill site near a Forerunner archive that contained a control terminal for the Sentinel defense network.{{Ref/Level|Id=Fighting Retreat|[[Halo Wars 2: Awakening the Nightmare]]|[[Fighting Retreat]]}} As the area was honeycombed with fissures filled with unstable explosive power, Pavium managed to use Banished Salvage Drills to start a chain reaction to burn away the majority of the Flood in the area, allowing Voridus to access the archive.{{Ref/Level|Id=Light the Fuse|[[Halo Wars 2: Awakening the Nightmare]][[Light the Fuse]]}} Inside, despite heavy Flood resistance, Voridus and his troops managed to reactivate the Sentinel defense network which began to cleanse the Flood.{{Ref/Level|Id=The Archive|[[Halo Wars 2: Awakening the Nightmare]]|[[The Archive]]}} However, by this time, the Flood had created a massive [[Proto-Gravemind]] in the area near High Charity that was in danger of becoming a full-fledged [[Gravemind]] while the threat from the Flood forced Atriox to pull back from positions he had established to fight the humans.{{Ref/Reuse|Manifestation}} This Proto-Gravemind retained a measure of the deceased Gravemind's intellect, but the Gravemind's new vessel had to feed and spread in order for the Gravemind to fully manifest once again.{{Ref/Game|Id=IH2|HW2|Detail=''[[Phoenix Logs]]'' - ''Proto-Gravemind''}} Voridus, Pavium and Banished forces, with the help of the Ark's defenses, in particular a [[Retriever Sentinel]], managed to kill the Proto-Gravemind before it could become a Gravemind.{{Ref/Reuse|Manifestation}}

With the Proto-Gravemind dead, the Flood threat was greatly diminished. Atriox ordered Pavium and Voridus to clean up their mess and the Banished joined a force of thousands of the Ark's Sentinels in containing the Flood outbreak and sealing off High Charity once more.{{Ref/Reuse|Manifestation}} The Banished and the Sentinels ultimately ended up "scouring" the Flood from High Charity's ruins.{{Ref/Novel|Id=Sacrifice|SoR|Detail=''[[Sacrifice]]''}}

The Flood is composed of a variety of specialized forms, each filling a different role in combat. The most basic Flood forms are the infection forms, which come in various types and infect hosts by hijacking their bodies and nervous systems.{{Ref/Game|Id=infectors|HW2|Detail=''Phoenix Log'' - ''Pod Infectors''}} Infected hosts then undergo a rapid series of transformations, becoming lethal combat forms. Combat forms are used to subdue and secure additional hosts for infection, and once they have outlived their usefulness as warriors, they transform into carrier forms, spreading the Flood by spreading more infection forms.

[[File:H2A Tadpole2.png|thumb|A Flood tadpole in a tank.]]

Pod infectors undergo a tadpole phase of development before growing into full physical maturity.<ref name="onefinaleffort">[http://halo.bungie.net/news/content.aspx?type=topnews&cid=25650 ''Bungie.net'': ''One Final Effort (4/16/2010)'']</ref>

[[File:HW2-SeederInfectors.png|thumb|A Seeder infector.]]

These large tanking forms can only be produced when a Flood outbreak has amassed considerable amounts of biomass and formed a Gravemind. These forms can take and deal massive amounts of damage.  

These flood forms can only be produced once a Flood outbreak has obtained enough biomass to form a Gravemind. These forms attack via bony spikes shot from it's back area, however is immobile.

 

These pure forms can only be produced if a Flood outbreak has gathered a large quantity of biomass to creat a Gravemind. These forms are very fast and agile.

[[File:HW2-Infestor.png|thumb|An Infester form.]]

These forms of the flood produce various Flood forms.  

[[File:HW2-Spawner.png|thumb|A Flood Spawner.]]

[[File:HW2-Burster.png|thumb|A Flood Burster.]]

[[File:HW2-Abomination Render.jpg|thumb|An Abomination.]]

[[File:Flood wall.jpg|thumb||150px|A wall covered in heavy Flood biomass: evidence of advanced infestation.]]

The first stage of a Flood outbreak is typically initiated by contact with Flood spores or even more reduced forms of Flood biomatter, should no extant Flood forms be present.<ref name="wp flood">[https://www.halowaypoint.com/en-us/universe/species/flood ''Halo Waypoint'': ''Flood'']</ref> Once a basic infestation has been established, Flood infection is typically carried out by [[Pod infector]]s, which are some of the most complex forms that can be generated from Flood biomass during the [[Feral Stage]]. Upon sighting a host, a Pod infector will leap for the most accessible area of the victim's torso; if the target is an armed human or other bipedal species, this is typically the left side of the chest below the neck, as most combatants will expose this area while shouldering their weapon.{{Ref/Book|AoBW}} Upon contact, the Pod infector will rasp away at clothing and flesh until it has burrowed into the chest cavity, mortally wounding them. As the host dies, the Pod infector extends tentacles that tap into the victim's spinal cord or other nerve center, forcing a "match" with the nerve signals previously produced by the host's living brain.  

The Flood infection then uses the absorbed biomass to create new masses of Flood Super Cells. As the Flood continues to use the resources of the host body, particularly its calcium reserves, the parasite's own cells build upon and augment the framework of the host to produce one of a variety of Feral-Stage Flood forms. This entire process, from the initial kill to total control over a fully mutated host body, takes only a matter of seconds.{{Ref/Book|AoH|Page=28}}{{Ref/Book|AoH|Page=50}}

The Flood infection utilizes the host's biological content as the fuel for its work, which is consumed and employed at prodigious speeds. From the beginning until the end of this complex, multi-step process, the Pod infector keeps the body "alive" by chemically isolating the host's dying brain, preventing the signals of brain death from reaching the rest of the body and causing it to shut down, which is an essential part of the parasite's mission of consuming and converting the host organism.{{Ref/Book|AoH|Page=48}}

While the Pod infectors are an effective and economical means of delivering infection, particularly against armored targets, the Flood can infect hosts through other means. [[Flood spore]]s, if inhaled, can transform a host into a combat form. This is the Flood's primary means of infection when an outbreak reaches a certain point: Flood spores will be deployed into a local biosphere in copious quantities, instantly overwhelming any unprotected biological targets.{{Ref/Novel|Id=s231|Sil|Page=231-232}} Flood conversion may also be initiated through contact with lacerated tissue, making any Flood form a potential vector for infection.{{Ref/Reuse|mona lisa}} Even the most basic form of Flood genetic material is virulent; once inserted into a host, it will mutate the host's DNA over the course of generations until it is capable of forming its own Flood super cells.{{Ref/Novel|Cry|Page=268-270}} When infected by these mechanisms, host organisms tend to mutate at a more restrained level that maintains the structure and form of the infected life form, albeit with a significant number of haphazardly placed tendrils, claws, and minor organs, such as sensory stalks.{{Ref/Comic|HGN|Page=19-54}}

Pod infectors selectively target species that possess sentient intelligence and are of sufficient biomass, and can infest intact or lightly wounded dead bodies. As a host organism is killed by the initial attack, Pod infectors use tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host's nervous system and gain control of their body, replacing its now-absent consciousness with the ravenous and voracious psyche of the Flood. Any useful information present within the memory centers of the host's brain, such as battle strategies and technical knowledge, is retained for use by the Flood. Despite this lingering of certain memories, no trace of the original mind remains; only a simplistic and primal urge to assimilate other species drives the organism after infection. However, the entirety of the host's mental content is seemingly absorbed and added to the whole, as the Gravemind has shown to be able to simulate its victims' consciousnesses within itself.{{Ref/Reuse|Gravemind}}{{Ref/Novel|Sil|Page=251}}

In isolated cases, such as when the Flood seek very specific information from an individual's mind, they utilize an alternate method of infestation that does not subdue the host's consciousness immediately despite mutating their physical form; this allows the Flood to "burrow" into the host's mind, eventually giving them access to the entire scope of the host's memories. This has only been specifically seen when the Flood sought information from Captain [[Jacob Keyes]], such as the location of Earth, or a starship that would free them from [[Installation 04]].{{Ref/Novel|TF|Page=175}} In addition, if the Pod infector that initiates assimilation is extremely aged or damaged somehow, the host may remain alive and conscious; Private [[Wallace Jenkins]] suffered such a fate, remaining alive and fully aware of his situation despite the mutation of his form, even exhibiting limited control over his body at times when the Flood instincts were dormant.  

{{Quote|The Flood changes everything. Not just flesh. Space itself is infected. That's the power the Precursors once had...  isn't it?|The [[Ur-Didact]] to the [[IsoDidact]].{{Ref/Novel|Id=s223|Sil|Page=223-224}}}}

Although the Flood is biological on the most basic level, as an outbreak advances, it becomes a highly powerful self-replicating swarm which will seek to harness any mechanism — philosophical, ideological or technological — to perpetuate itself and its goals. These aspects were most prominently demonstrated during the two major outbreaks in the Forerunner era, particularly the [[Forerunner-Flood war]].{{Ref/Novel|Id=s214|Sil|Page=214-215}}

The Flood is able to use any technology it has assimilated, often to a superior degree than the original creators of said technology due to its vast base of knowledge and intellect, as evidenced by its ability to transition the [[UNSC frigate]] ''[[UNSC In Amber Clad|In Amber Clad]]'' into ''[[High Charity]]'' with a precision slipspace jump normally unheard of with human ships. Similarly, it was capable of tapping into [[Installation 05]]'s [[teleportation grid]] and utilizing it with great accuracy.{{Ref/Reuse|Gravemind}}

There is no confirmed way to effectively stop a Flood infection once Flood Super Cells have entered a host. The only exception is too rare and factually suspect to be considered viable: the human disease "[[Boren's Syndrome]]", a neurological condition where radiological exposure causes electrical anomalies in the host nervous system, allegedly renders the host immune to infection while living. The neural frequencies of the host are "scrambled", apparently without negative consequences to the host. However, during Flood infection, the Pod infector is unable to tap into the host's nervous system, rendering it incapable of proceeding to large-scale physiological mutation. An interesting side effect of this failed infection is that Flood DNA and cells appear to be harmlessly integrated into the host's body. This seems to provide the host with increased regenerative capabilities, although this has not been independently verified. While Boren's syndrome left untreated may render the victim immune to the Flood, leaving it untreated almost always equates to death within a couple of years. There is said to be a "one in a billion" chance that research into the disease could produce a cure.{{Ref/Novel|FS|Page=244-246 (2001 edition)}} However, the accuracy of this information remains highly dubious, as most of it is simply a hypothesis devised by [[Catherine Halsey|Dr. Halsey]] to explain how [[Avery Johnson|Avery J. Johnson]] survived Flood infection. Johnson's "Boren's Syndrome" was actually a story invented to cover his past in the [[ORION Project]]: his symptoms were side-effects of his experimental enhancements, not a neurological disease.{{Ref/Comic|HGN|Detail=[[Halo Graphic Novel, Page 122|page 122]]}}

While the Forerunners could not find a cure for Flood infection, the [[Composer]] was designed to eliminate its effects by [[Mind transfer|extracting]] a Flood victim's consciousness into digital form and then re-inserting it in an artificially-constructed body devoid of infection. However, the Forerunners never perfected this process and any imprinted bodies died soon after,{{Ref/Novel|Sil|Page=40}} due to the Flood somehow resuming the infection in the new body. The Forerunners attempted to use various means to slow down this process, including a specially designed harness of mesh and wires, which would apparently counteract the infection. The victim would retain most or all of their mental faculties and self-control, although their body would continue to be mutated by the Flood. This countermeasure was only temporary, however, and the victim would eventually either decay and die or be fully subsumed by the Flood. Several Forerunners on Installation 07 were subjected to this countermeasure.{{Ref/Novel|Pri|Page=268}}{{Ref/Novel|Pri|Page=322-323}}

Environmental stimuli seem to have an effect on Flood metabolism. In the event of a Flood outbreak, Halo installations create a [[Quarantine zone (location)|quarantine zone]] around the affected area,{{Ref/Level|H2|[[Quarantine Zone]]}} inside which the ambient temperature is drastically lowered. [[Cold Storage|At least one]] [[Flood research facility]] used low temperatures to keep its specimens inactive and dormant as it studied them.{{Ref/Game|H3|Detail=multiplayer level ''[[Cold Storage]]''}} At least [[Control Room (Installation 04)|one]] of the Halo installations' [[Control Room]]s was located in a snowy environment, possibly to hinder escaped Flood in their attempts to reach it.

The Flood are also vulnerable to temperatures on the other end of the spectrum. [[plasma weaponry|plasma weapons]] will easily burn through Flood tissue; the Forerunners incorporated directed-energy weapons into their [[Sentinel]]s, indicating that they had determined high-enthalpy energy weapons to be the best 'basic countermeasure' available to fight the Flood. Known examples of Forerunner personal weaponry appear to be geared toward the disintegration of organic matter,{{Ref/Game|H4}} also reflecting the Flood's vulnerability to extremely high temperatures. The UNSC has taken advantage of this weakness as well, making heavy use of [[flamethrowers]] in their engagements with the Flood.

When engaging heavily entrenched Flood structures such as [[Flood colony|colonies]] and [[Cocooned base]]s, orbital bombardment is usually a preferred method of elimination as direct attacks can be extremely costly.{{Ref/Game|HW2|Detail=''[[Phoenix Logs]]'' - ''Cocooned Base''}}

During the [[human-Forerunner wars]], humanity (who at the time was a [[Prehistoric human civilization|technologically advanced, spacefaring race]]) believed they had discovered another way to defeat the Flood. Using genetic engineering, a third of the human population was altered to carry a set of specially designed genes. This group of humans was then allowed to be infected by the Flood. Once the genes were exposed to Flood DNA, they triggered a cascade of cell destruction that spread throughout all Flood biomatter. The majority of the Flood died off as a result, although they would return in full force ten thousand years later. All information regarding this "cure" was destroyed by the humans as a final act of revenge against the Forerunners, leaving them unprepared for the second Flood strike.{{Ref/Novel|Id=cry267|Cry|Page=267-272}}

Flood combat forms have been sighted crudely operating {{Pattern|Karo'etba|Ghost}}s, [[M12 Warthog|Warthog]]s, [[Zurdo-pattern Wraith|Wraiths]], and [[Scorpion]]s in ground operations, although with slow deliberation and clumsy driving and accuracy. The assimilated crewmen of the [[UNSC frigate]] ''[[UNSC In Amber Clad|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 [[Installation 05|Delta Halo]] to ''High Charity'' and then crashing the vessel into a tower, releasing Pelican dropships to rain down upon the Covenant capital.

While one of the Flood's main strengths lies in the decentralization of its compound mind and ability to be reborn from even the most carefully reduced remnant, it is nonetheless known to develop and assume a certain "core" Gravemind as an embodiment for its strategic intelligence. The Forerunners observed that the Flood exhibits a seemingly instinctive tendency to recoil and concentrate all of its otherwise distributed assets around this core when threatened on a large scale. A threat to the Flood's existence major enough to prompt such action was the imminent firing of the Halos, at which point the Flood gathered all of its available assets and launched an attack on the Ark.{{Ref/Game|H3|Detail=[[Terminal (Halo 3)|Terminal 3]]}}

The Flood is very effective on a psychological level, as its parasitic nature and gruesome appearance alone are a source of horror for most of its enemies and thus serve as powerful demoralizers. It is not uncommon for survivors of Flood encounters to be severely traumatized, often to the point of delirium, after witnessing their comrades get infected or being subsequently forced to kill said infected individuals.{{Ref/Level|CE|[[343 Guilty Spark (level)|343 Guilty Spark]]|Detail=[[Paranoid Marine]]}}{{Ref/Level|H3|[[Floodgate]]|Detail=[[Suicidal Marine]]}}

*Some combat forms derived from Elites still have functional energy shields. However, they never possess active camouflage, as some in ''Halo: Combat Evolved's'' [[The Maw |final level]] did.

*Sticking a carrier form with a {{Pattern|Anskum|plasma grenade}} or [[Spike Grenade]] will kill both the carrier form and the pod infectors it carries. This is also true of a [[Kemuksuru|supercombine explosion]].

*Many new forms, such as [[Flood bomber form|bomber forms]], were created for ''Halo Wars'' to expand the "roster" of Flood units. The official explanation is that these forms were created from creatures indigenous to [[Trove]].

[[File:HCE EarliestFlood Concept.jpg|thumb|The earliest extant concept art of the Flood, by Robert McLees.]]

* The Flood's AI routines in ''[[Halo: Combat Evolved]]'' were originally intended to be "every bit as complicated as the Covenant", but full implementation was prevented by time constraints.<ref>''EDGE'' magazine - September 2003, ''page 109''</ref>

*The Flood were inspired by Christopher Rowley's [http://www.christopherrowley.net/novels/VangSeries.html The Vang].<ref>[http://www.gamespot.com/xbox360/action/halo3/news.html?sid=6162009&mode=previews&msg_sort=1&cpage=11 ''Gamespot'': Halo - Q&A]</ref>

*An alien virus named "[[tardis:The Flood (The Waters of Mars)|The Flood]]" appeared in a 2009 special episode of the British science fiction television show ''[[wikipedia:Doctor Who|Doctor Who]]''. Although like its Halo counterpart, it is hinted to be intelligent, and is extremely virulent, transforming its victims physically, the Flood found on Mars literally uses water to transmit itself.<ref>''Doctor Who'', ''[[tardis:The Waters of Mars|The Waters of Mars]]''</ref> The previous episode, also a special, involved the alien planet "San Helios", a name very similar to ''Halo's'' [[Sanghelios]].<ref>''Doctor Who'', ''[[tardis:Planet of the Dead|Planet of the Dead]]''</ref>