The Flood is a fictional alien life form in the video games Halo and Halo 2.
Overview
The Flood is a parasitic virulent life form which is spread by infected carrier hosts. The Flood appears in three primary forms: Infection, Combat and Carrier forms, but also have more mysterious forms like the Brain form and the Juggernaut form. All of these have an attraction to any lifeform apart from themselves and will try to infect and attack any they encounter. The lifecycle of the Flood begins at Infection form as a small frail gas bloated fiend that leaps upon a host and drives in three barbed tentacles. (Although the small spores that some Flood are seen to release could be used to infect objects as well. This can be seen with the Flood-Infected trees on the Multiplayer map, Backwash) From here, the host is paralysed and loses consciousness. The infection form gores itself deep into its host, usually in the chest. For a while it plays with its new body, and over time mutates it by changing hormones and replacing the host's nervous system with its own grotesque tentacles. The host is never fed, and carelessly treated. Often limbs become infected and begin to rot. Vital organs are liquified and the neck is broken and head disused and eventually falls off. If the host is of suitable size eg. Human or Elite, long whip like tentacles emerge from the arms as weapons. If not of suitable size, then the next stage of mutation occurs early. The body becomes so bloated and rotten that it may no longer be used as a Combat form. Smaller Infection forms grow within the ball of flesh, as it swells with gas and juice. Eventually the Carrier form falls over or simply gives up and explodes. Roughly 4-9 infection forms are emitted and continue the cycle.
There is speculation into many other forms such as spores, unknown and shapeless flesh lumps, brain forms that strip information, Gravemind and even the Juggernaught form.
The origin of the Flood remains unknown, lost in the immense amount of time since those events took place. What is known is that the Forerunners constructed gas giant research facilities and later huge "Fortress World" installations (called "Halos" by the Covenant) to contain and study the Flood. In the event that the Flood was released, which it was, the Halos were designed to destroy all sentient life in the galaxy. The thinking behind this is that if the Flood has no food then it will eventually starve to death, allowing the galaxy to slowly re-populate and preventing the Flood from spreading to even more galaxies throughout the Universe. In the sequel, Halo 2, some levels took place onboard a Forerunner facility. Throughout the levels were many small, decorative pedestals supporting objects that are similar in shape and colour to various forms of Flood. These decorations are repeated many times, too precisely to have been created by the Flood, and are probably intentional references to it.
Flood forms
Infection forms
The Infection forms are small, tentacled creatures which have their own defined biological framework, which are not dependent upon a host. An infection form will seek out any life of capable bio-mass and calcium deposits to sustain itself, and proceed to attempt to use the creature as a host, by tapping into the spinal system, suppressing the host's consciousness, embedding itself in the thoracic cavity, and releasing spores which cause the host to mutate. (343 Guilty Spark mentions these "spores" during his own synopsis of the Flood life-cycle, which may be either another term for the Infection form, or an even more base form of the Flood.) At this point, the infected creature mutates into a possible one of four other forms. The Infection Form Flood parasites attack in a flood-like swarm that rapidly advances on host organisms, then physically attach themselves to a host as they try to burrow into its chest. Once inside the organism will begin to modify the host's genetic code and take over. The Infection form cannot attach to a host if the host is protected by a personal energy shield; contact with the shield will make them explode with a distinct "pop" sound and do minimal damage. However, if several dozen try to attack a shielded host they will gradually deplete their shields to the point that they are vulnerable. Thus, soldiers such as the human SPARTAN soldiers and the Covenant Elite are able to at least have some protection against the Flood and mount a counter attack, but unshielded troops (the majority of both armies) such as UNSC Marines and Covenant Grunts, Jackals, Hunters (with enough time or Infection forms) Drones, and Brutes are quickly and easily obliterated by an attack from the Infection form Flood.
Combat forms
If the host is strong or deemed "useful" by the parasitic infection form, then the host will be converted into a warrior or worker form, used in defensive/offensive situations as well as maintaining and repairing machinery (In Halo 2, the Flood had the ability to pilot and repair vehicles, which suggests that the Flood utilize the memories and knowledge of their hosts). These warrior or combat forms are extremely strong and agile and can jump an extremely far distance, with the Covenant Elite-derived combat forms sometimes being able to use the Covenant Elite technology remaining and even the energy swords of their hosts. These forms can use all UNSC and Covenant weaponry, except the Fuel Rod Gun. Combat Flood are also capable of unarmed combat, striking with whip-like tentacles protruding from the arms of the host. They are also known to play dead after being shot by a weapon other than the Shotgun, Needler, or Rocket Launcher. The only surefire way to kill them is shooting their infection form residing within their chest area (it must be noted however, that surrounding infection forms have been known to enter the downed combat form and 'bring it back to life', therefore it is advisable in an area where infection forms are in the viscinity to completely destroy the combat form using a close range shotgun blast). They also yield grenades within their bodies, dropped when they fall down when shot, they also have tendencies to commit friendly fire, and look at grenades that are set off, considering the Flood are somewhat simple-minded.
UNSC Marines and Covenant Elites are combat forms. To tell the difference, the Covenant type are larger, and drop Plasma Grenades when shot. The Human type is smaller and a lot more frail in their appearance, and drop Fragmentation Grenades. The Human form is also more partial to Human weapons, although this is difficult to monitor, and could be because they had these weapons when they were infected.
Carrier forms
The second possible form an infected host can take is that of the carrier, which presumably only happens to creatures who are physically or mentally unsuitable to be Combat forms. The Carriers grow large sacs which contain several infection forms. When a Carrier is in close proximity to any number of suitable hosts, it triggers some manner of biochemical reaction, causing the sacs to explode and spread the infection forms in order to infect more hosts. This continues the Flood life cycle. Also, as the Flood do not care for their physical forms, older and worn Combat flood can be turned into Carriers once they have outlived their usefulness. A disadvantage is that the explosion of their bodies can set off grenades, so a veteran player can shoot them at the right moment to set of a series of detonations, which can kill the Infection Forms released as well as other opponents. In addition, the explosions from the Carrier Forms themselves act as grenades, often causing a powerful enough force to send other nearby Combat and Carrier Forms flying. Firing directly at Carriers in a group of Flood can cause chain reaction explosions with other Carriers, thus helping to eliminate potential threats.
The Covenant Grunts and Jackals serve as Carrier forms.
Juggernaut
The Flood Juggernaut is one of the most enigmatic of the flood forms. They were initially planned to be encountered in Halo 2, but it was for some reason rejected by Bungie. The Juggernaut seemed likely to be some form of boss or strong enemy (such as a Hunter), although its purpose is never revealed. It is physically taller than a Hunter and has a physical form resembling that of a carrier, but with tall legs and unusually long tentacles. Their abilities are unknown, although its large tentacles suggest that it has a powerful melee attack. The Juggernaut was previously unknown to the Halo community until the code and models for the creature were found buried in the Halo 2 CD.
With not much being known about the Juggernaut, it is difficult to assume what its original host was. Some speculate that it is a flood-infected Hunter, though due to the fact that Hunters are colonial organisms it makes infection seem highly unlikely. It could be a combat form mutating further into a carrier. Others believe it is the infected form of a Drinol, another enemy that never made its debut in Halo 2.
Brain forms
Brain forms are slightly infected, but not killed, like the other forms. They are used to collect information on important things, such as codes, planets, locations and species. Captain Keyes was infected, and turned into a Brain Form. They look like blobs that have other organisms' body parts for arms and legs. Over time the "Brain form" strips memories and information from its host, but is only able to take memories and information that the host is currently thinking of. Many other individuals are also absorbed into the brain form, but are simply used as food sources. All body tissues and bones are soon reduced to fluid, and is digested as the brain slowly absorbs the information. Eventually the brain form is used to create more infection forms.
Command forms
The command forms, due to their limited exposure in the Halo trilogy, are not well understood. Their purpose is not exactly known. The Gravemind from Halo 2 is also believed to be a command form over the Flood on Installation 05 (Delta Halo).
Flood details
Late in Halo: Combat Evolved it is revealed by 343 Guilty Spark that there were originally seven Halos; six currently remain after one (Installation 04) was destroyed in the first game. As each Halo has a firing range of 25,000 light years, there are still enough Halos to destroy all sentient life in the galaxy.
The Flood on Installation 04 were completely destroyed in Halo: Combat Evolved and in Halo 2 they returned with a vengeance with the ability to use vehicles and command starships which could take them anywhere in the galaxy. Their rulers are apparently Gravemind, who has a base of operations beneath Delta Halo's Library and on High Charity, and its offshoot, a Command/Pilot/Brain form, formerly Captain Jacob Keyes who occupies the second Covenant cruiser in Halo: Combat Evolved. It is not currently known if there is a Gravemind in each Halo, or if the Gravemind encountered on Delta Halo is the controller of all Flood. The Gravemind is also thought to be the collection of the floods minds, not the leader of the flood.
The name "The Flood" was most probably given due to the species' sheer force of numbers. However, it may also be a reference to the Great Flood which wiped out the sinful in the Biblical story of Noah's Ark. This is supported by the existence of an enigmatic location known as "The Ark" which can remotely control the Halos.