Flood

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

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

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

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

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

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

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

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

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

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

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

Infection Form


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

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

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

Combat Form


After an infection form has obtained a frequency match for the host's neural system, it begins to rewrite the very DNA of the body it has infected. Once fully transformed, the Flood infection is in total control of the body. Unfortunately for the host, he or she stays alive throughout the transformation process and is even conscious of his environment as the Flood takes over the body. Once completely infected, the Flood becomes what is known as a Combat form. Any body of sufficient size, capable of carrying a weapon or exhibits a possible use in combat is transformed into one of these monstrosities. The Combat form grows powerful, whip-like tendrils on its arms, and will use them as close combat weapons, or if it loses the ranged weapon it carries.

The Flood infection is capable of not only controlling the host's body, but of also tapping into and utilizing its memories. Most often for the purpose of finding relevant information about the use of weapons and equipment. After the Combat form has suffered a sufficient level of punishment as to be no longer effective, it mutates into a carrier form. A biological incubator for more infection forms. Eventually a combat form will mutate into a carrier, regardless of damage, to perpetuate the Flood species.

Carrier Form
When an Infection Form takes over a body that is unsuitable for combat, or once an infected form is damaged beyond use, it becomes a Carrier Form. Grunts and Jackals are immediately turned into this form upon infection, due to their weaker physiology they are not deemed viable to become Combat Forms. Carrier Forms perpetuate the Flood species by acting as incubators for new Infection Forms and keeping them safe until they're fully developed. Once the Infection Forms are ready, the carrier simply explodes spontaneously, sending the new Infection Forms flying into the air. As soon as they are released, they begin searching for new hosts.

Brain Form
One of the rarest types of Flood is the Brain Form. A Brain Form is unlike all other Flood forms since it lacks any sort of mobility. While the Infection, Combat, and Carrier forms are dangerous and always hostile, the Brain Form is passive to all but the host within its bloated form. When the Flood attempt to control a starship or interface with a computer system, a Brain Form is created. It normally contains the body of an individual who has knowledge necessary for operation of said vessel. Like the Combat form, it can read the thoughts and memories of its host provided he or she is still alive, and it will sift through every memory until it finds what it needs. If it cannot find the information it requires, it will direct other Flood to bring it a new body to search while it consumes the last body. It interfaces with computers the same way an Infection Form infects a host, through tendrils that attach themselves to the computer's "neural" system. Note, in the picture to the right, 3 separate absorbed humans can be seen, one being Captain Keyes. Captain Keyes got turned in to a flood on the 9th level and everyone found out he was a flood when the Master Chief found him.

Prophet Form
Prophet Form is a term used to describe Covenant Prophets that have been infected by the Flood. Due to their unusual nature, they are not thought of as Combat Forms in the way infected Humans and Elites are (note that the term "Prophet Form" has not been officially used as canon to date). It is also worth noting that both "Prophet forms" seen thus far had only recently been infected, and may have required more time to be assimilated in the conventional way.

It can be concluded based on the concepts illustrated in the Halo: The Flood and the Halo Graphic Novel that figures such as Prophets or military commanders would be spared the common assimilation procedures for the purpose of preserving the individuals knowledge for later use.

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

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

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

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

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

Infection


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

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

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

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

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

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

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

Halo: Combat Evolved and Halo 2
The Flood have no obvious strategy. They fight without fear of death or injuries (a lone Combat Form will sometimes sacrifice itself to advance the cause of the battle). Speculation of such behavior could be due to the collective intelligence that all Flood share, and destruction could simply is to "sleep". They attack by finding a group of enemies and then send everything they have at them hoping for victory through attrition. The Flood have shown a preference for close combat. Their less than exemplary motor skills make them poor shots, but they have powerful whipping tentacles that cause great damage at close range. In the melee it is not uncommon for Flood to be destroyed by one another in the madness of combat. The Flood does not worry about such collateral damage since disabled Flood bodies can be reanimated by Infection forms. Dead Flood can only be permanently disabled by being completely destroyed. A tactic that the flood sometimes use is to send in waves of combat forms as cannon fodder to weaken the enemy forces, or drain the enemies ammuntition supplies and then come in with waves of infection forms to claim the vulnerable enemies. One of the best ways to combat the flood is with a rapid-fire human weapon, such as the assault rifle or smg, paired with a powerful one-hit weapon, such as the shotgun or energy sword. The assault rifle/smg works well agains infecton and carrier forms and the shotgun/energy sword destroys combat forms in one hit.

The Flood is a very intelligent race, gaining the knowledge of each host they infect. In addition, they are under the control of the Flood "puppet master" The Gravemind (this is revealed in the Halo 3 plot summary on bungie.net).

Halo Graphic Novel
It should be noted that the Flood assimilation and behavior in HGN was somewhat different from the games. It is stated by the Legate that the Flood is mentioned in the holy texts of the Forerunner and they are one of the tests before the Great Journey. He also stated that the Flood were gathering bodies and body parts, giving them to the larger organism which demonstrated some form of intelligence and the general consumption of animal matter. This is similar to the events in Halo 1 where the John-117 boards a Covenant ship to find Captain Keyes. Cortana notes the Flood are gathering bodies and it is most likely for the Brain Form that was found on the bridge.

Also, the rate of infection in the Covenant forces was extremely fast. Within minutes of contact, many Elites and Grunts were mutated quickly into Flood forms. The mutations occurred randomly and were not uniform in any way. Sometimes the Elites were transformed into larger forms than the Combat Form. Other times they developed a Flood-like carapace and on many occasions on the Infinite Succor, the Flood used tentacles as their primary weapon similar to the Gravemind. However, when Rtas 'Vadumee battled his comrade Kusovai, Kusovai continued to mutate throughout the duration of the battle, suggesting the possibility that over time any infected host can mutate further and further.

In addition, the Flood can instantly absorb knowledge from an infected host into their collective mind. When Rtas' teammates were assimilated, the Flood could open any doors and access any system on the ship right after the teammates were assimilated. When the Flood then assimilated the Legate, the Flood reflected the knowledge of him to Rtas while he was in engineering, mocking him and stating that "The Forerunner not defeat us...what chance you?" It would appear that the threat of the Flood is far greater than previously assumed.

Trivia

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