Flood

The Flood (sometimes called The Parasite by the Covenant) is a species of virulent, parasitic organisms that thrive by consuming other carbon-based, sentient lifeforms of sufficient biomass. . The Flood are not native to our galaxy, and their origin and reason for coming here are unknown.

Introduction
The Flood is one of the main antagonists and the most dangerous enemy in The Halo Universe. They are the reason the Halo's were built. The Flood's only goal is to infect every sentient organism in the galaxy.

Next to nothing is known about their origins. The Forerunners believed the flood to be extragalactic (that they came from another galaxy). However, their potential for destruction is not in doubt. 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.

This much is known; the Flood were kept by the Forerunners for scientific study on the Halos as well as other structures. Oddly enough the Halos are an immense weapons array designed for the sole purpose of wiping out the Flood's food, not the Flood itself. Although the intention was such the fact that the Flood still exists indicates that the Forerunner's plan may have failed.

Infection
The Flood takes over the bodies of hosts and assimilates them into the Flood. This is done via Flood Infection Forms. 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, taking over brain functions to prevent resistance of any kind. After this time, the original mind is erased memory by memory, until nothing is left but a "hungry" Flood form. Separate tendrils release aggressive spores laced with Flood DNA into the host, causing the body to mutate into a Combat Form or a Carrier Form.

The Effects of Infection on the Host's Body
The book Halo: The Flood describes the effects of assimilation on the human body in great detail.
 * First the host's extremities grow numb and their lungs feel heavy. Then the greater Flood intelligence begins to take over the mind.
 * The host's vision seems to be "a gauzy white haze", but the host can make out shapes . The host can hear a low droning through the ordeal.
 * The limbs feel to the host as if they are made of a spongy liquid. Their lungs itch as well.
 * The Flood controlled Combat Form can generate "tentacles" at a rather rapid pace, in total disregard for the host's skeletal structure.
 * There also seems to be an agent released into the host's blood stream that softens bone tissue.
 * Combat forms also seem to resemble their host before the infection to a degree.

The only known way to effectively stop a Flood infection is to have Boren's Syndrome, the radiation-caused disease that Sergeant Johnson possesses. Boren's Syndrome does not completely deny the infection, however, which is proven by the fact that Johnson has an increased, but limited, regeneration ability. However there appears to be some debate as to the validity of this condition. There is some evidence to support the accusation that Boren's Syndrome was in fact a cover for Sgt. Johnson's speculated status as a Spartan I.

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). 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.

Infection Form
The Flood exists as multiple organisms but the largest it can get without requiring extra bio-mass is the Infection Form. This form, while not immediately recognizable as a threat from a distance or on its own is dangerous nonetheless. In large swarms this form compensates for its fragility with sheer weight of numbers. Large groups can rapidly overwhelm much larger creatures.

Over extended periods of time the Flood infection bacteria seems to become less powerful. These weaknesses allow for the host to control their own thoughts and actions to a degree. A known case of this was PFC Jenkins.

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.

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.