The Flood

The Flood is a parasitic virulent life form, known for their vast numbers and their ability to take over the bodies of sentient life.

History
Like many things about the Flood, their origin is a mystery. It is unknown where they came from, or how they were moved to the Halos. What is known is that the Forerunner somehow came in contact with the Flood, and that the Flood were considered so great a threat that the Forerunner created the Halos. The Halos were a last-resort weapon designed to kill all sentient life in the galaxy, therefore starving the flood to death.

The Halos were fired once, and the Flood were buried in the Halos somehow. It wouldn't be until thousands of years later that the Flood returned. The Covenant, believing that triggering the Halos would be the key to starting the "Great Journey," found Installation 04 and began exploring its depths. Unfortunately, they found the Flood. The Covenant attempted to suppress and seal them, but a team of UNSC marines unwittingly unsealed the Flood, sealing the fate of Installation 04. The Flood expanded across the ring world, attempting to spread far and wide. There were two recorded attempts to board Covenant ships and leave the Halo.



The first was when a force of Flood combat forms managed to take over the Covenant supply/agriculture ship Infinite Succor. A strike team led by Rtos 'Vadumee managed to destroy the ship, with the Flood on it.

The second attempt was when the flood tried to board and leave with the Truth and Reconciliation. This attempt was less successful, and the Flood did not manage to leave with the Truth and Reconciliation before Installation 04 was destroyed by Master Cheif and Cortana. (Halo: Combat Evolved)

The Flood somehow managed to make it to the Forerunner station in Threshold, the gas giant that Installation 04 orbited. They overran the station before it was plunged into the heart of the planet.

The second outbreak was on Installation 05, also known as Delta Halo. This outbreak by the Flood was far more advanced, with the Flood managing to take over High Charity, using UNSC Pelican drop ships to board the giant floating city. On Delta Halo, the Gravemind revealed himself to Master Cheif, Cortana, and the Arbiter. Eventually, the Gravemind rooted himself in High Charity, and the flood took control. It is unknown as to whether or not the Flood were eventually destroyed. (Halo 2)

Biology and Life Cycle
The Flood appears in three primary forms: Infection, Combat and Carrier forms, but also have more mysterious forms such as the "Brain" form. The flood will consume all sentient life it comes in contact with, and can mutate its hosts into various forms to help it do so.

Flood forms can create a creature to organize the Flood, and on the Infinite Succor, they were seen building such a creature, bringing bodies for it to absorb. What this creature is, is unknown. There is a possibility that it could be a sort of "Brain" form (see below), or perhaps even a Gravemind type of creature.

Infection forms
The infection forms are small, tentacled creatures which have their own defined biological framework, and therefore are not dependent upon a host. Individually they are weak and fragile; one can kill an Infection form simply by squeezing it. However, their tendency to swarm in vast numbers makes up for their delicacy.

An infection form will seek out any sentient life of capable bio-mass and calcium deposits to sustain itself, and proceed to attempt to use the creature as a host. The infection form parasites attack in a flood-like swarm (hence the Flood moniker) 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 does this 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. At this point, the infected creature can mutate into a variety of other flood forms.

Combat forms
If the Flood finds a suitable body, it will infect the body and become a Combat form, which will attack any sentient life it comes in contact with.

When the flood infects an animal, living or dead, it absorbs the knowledge of its host. This knowledge can be communicated to other flood using unknown methods. This was demonstrated aboard the Infinite Succor, a Flood-infested Covenant ship. When the Covenant sent a strike team to the Infinite Succor, the Flood managed to capture a few Elite units. After absorbing the knowledge of the Elite, the Flood on the ship became more organized, setting up patrols and creating defensive positions.

Combat forms are extremely strong and agile. They also gain the ability to jump very long and very high distances. The Combat units are able to use weapons that their hosts were able to use, and gain any kind of advantage their host had biologically. The Elite-derived combat formsare sometimes able to use the energy shielding technology with their remaining Elite armor.

Combat Flood are also capable of unarmed combat, striking with whip-like tentacles protruding from the arms of the host. These tentacles can grow out of the arm at an alarming rate, simply breaking the nearby bones and displacing them.

Flood combat forms totally ignore any threat to themselves and those fighting alongside them and will often kill themselves to kill an opponent. Most of the time, Combat forms fight until their bodies are no longer capable of doing so, at which point they are either discarded or possibly turned into Carrier forms.

Carrier forms
An infected host can also mutate into a Carrier form. 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.

It is unknown what the criteria are for a host to become a carrier form. All answers are speculative at best.

Brain forms
"Brain" forms are large, spongy Flood forms which resemble a large bag of flesh with tentacles. Presumably, they are used to collect information on important things, such as codes, planet locations and information about the species that the Flood can infect.

Captain Jacob Keyes was assimilated by this Flood form. Over time the Brain form strips memories and information from its victims, but is not able to take memories that the victim is trying to hide from it (in Keyes's case, the location of Earth). All body tissues and bones are soon reduced to fluid, and are digested as the brain slowly absorbs the information. It is unknown how this information is communicated to other Flood forms, or if it is at all. (Halo: The Flood)

Gravemind
The Gravemind, and its place in the Flood, is enigmatic. The Gravemind is intelligent, and can communicate vocally. It also seems to be able to interact with machinery, as it can use Halo's teleportation grid.

Flood details
"The Flood" were probably named as such due to their overwhelming 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 enormous, enigmatic Forerunner structure believed to be located under the ruins of Mombassa's outskirts known as "The Ark" which can remotely control the Halos; the only thing that saved Noah from the Flood.

The Flood can terraform the atmosphere to suit them, as was seen on High Charity. Fleshy growths were attached to the walls, and the air became damp and gaseous. The Gravemind also took residence, and was able to use a Covenant speaker system to broadcast messages. (Halo 2)