The Flood, designated as LF.Xx.3273 by the Forerunners (Latin Inferi redivivus meaning "the dead reincarnated") referred to as the Parasite and the infection by the Covenant, the Sickness by humans, and Species 6 by the Borg, is a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the Milky Way galaxy - including the vast majority of Forerunners - during the Forerunner-Flood war in the ancient past, prompting the activation of the galaxy-sterilizing Halo Array in 97,445 BCE. Originating as a corrupted form of the universe's most ancient custodians, the Precursors, 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.

All Flood forms share a single consciousness, which becomes progressively more intelligent and sophisticated with every sentient host that is assimilated. When enough hosts have been infected, the Flood consciousness becomes a Gravemind - a self-aware, highly intelligent entity that strategically commands all Flood forms in an outbreak. Due to this all-encompassing collective consciousness, the Flood as a whole could be more accurately described as a single, transsentient macro-organism rather than a collection of individuals directed by one or more controlling beings. This organism has sometimes referred to itself as the Primordial, although the actual distinction between the Primordial and any given Gravemind, if indeed there is any, is unknown. The nature of the Flood's collective consciousness could be likened to a socialist utopia, due to the fact that the Flood act as a unified entity, with no individuality that would be present in other, more traditional species; each component of the Flood meta-organism works tirelessly for the advancement of their species.

The Flood presents a highly variable and unconventional threat in combat, as it can infect and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are the greatest threat to the existence of life (or, more accurately, biodiversity) in the universe.

Biology Edit

The Flood is an anomaly in the normal biology of lifeforms; their mode of growth and reproduction would make them incompatible with any natural ecosystem. They are extremely adaptive, and transform hosts to suit the species' current needs.

Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater and in the vacuum of space, the Flood can withstand the varied environments of all known inhabited planets. However, the Flood seem to thrive the most in moist and humid areas; it is thought that the extremely cold artificial weather patterns near certain important areas of the Halo rings are intended to hinder the Flood. In order to give themselves ideal living conditions, the Flood will transform the environment to better suit their needs, altering the atmosphere and covering solid surfaces with Flood biomass.

All Flood biomatter is made of a unique, undifferentiated "Super Cell", which relays information and coordinates the cellular assembly like conventional neural cells. It is also capable of flexing and moving in coordination with other cells, and can be arranged to mimic any bodily organ that is required for the Flood's use. This combination of traits has caused some to refer to it as "thinking muscle."

The Flood Super Cell physically resembles both neuron and glial cells in structure, with a central cell body and many tendril-like, branching structures.

In the absence of a Gravemind, the Flood possesses only basic coordination; they are incapable of forming complex strategies, and focus solely on attacking and infecting nearby sentient organisms. This stage of a Flood outbreak is known as the Feral Stage. At this stage, the Flood seek to create an assembly of biomass, calcium, and nervous system reserves, which leads to the creation of a Flood hive and the beginnings of a proto-Gravemind.

Infection Edit

In order to reproduce and grow in biomass, the Flood needs to infect and assimilate other life-forms. While capable of infecting nearly any organism, the Flood shows a strong preference for sentient life-forms, as assimilating their cerebral tissue allows the Flood to increase its overall intelligence.

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. Once a basic infestation has been established, Flood infection is typically carried out by pod infectors, 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. 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.

To begin the physiological conversion of a host, the pod infector injects encapsulated Flood Super Cells into the body. As it does so, the form analyzes the entire genetic code of its host, allowing the Flood to determine the most effective use of the victim. Simultaneously, the inserted Flood cells capture and break down the host body's cells into organic raw material, which is then absorbed and assimilated into the attacking Flood cells.

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.

The infection process is not limited to living hosts; organisms that are already deceased are also susceptible to infection. As long as the host has not decomposed to the point where the body and nervous system have completely deteriorated, the Pod infector can infect and convert the host.

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.

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. The original infection form used by the Leviathans was that of the Markers, neural physics-powered objects which transform dead biomass into material ready to be made of use by the collective. Later evolutionary stages of the infection yielded other methods of infection. Flood spores, 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. Flood conversion may also be initiated through contact with lacerated tissue, making any Flood form a potential vector for infection. 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. 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.

Neurological Edit

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 generate its victims' consciousnesses within itself.

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. 

When the Flood have amassed enough sentient hosts to form a Gravemind, then their behavior becomes far more coordinated, and the Flood collective becomes much more deadly as a result. In addition to its neural physics abilities, the Gravemind may also physically speak directly through combat forms and pure forms under its control.

Physiological Edit

While baseline neurological assimilation is largely the same regardless of host species, the Flood infection process includes physiological transformations, which vary depending on the host and the Flood's present need for specialized combatants.

Upon being infected, higher-level species capable of combat are typically transformed into combat forms. The host's physiology is completely "rewritten" during the infection process: organ-based systems are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible and brute force incapacitation difficult. Due to the lack of a central nervous system, decapitation will neither stop or slow a combat form.

Furthermore, munitions with high speed and penetration, such as sniper rifle rounds, are usually ineffective against combat forms. Picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without dealing any major damage.

To facilitate melee combat, combat forms generate tentacles and claw-like structures, which protrude from the host's arms. Generally, a single arm develops into a clawed appendage, while the other retains the capacity to manipulate objects, such as firearms. The rapid growth of these structures tends to push aside and break apart the host's hands and forearms.

Combat forms can strike with devastating force, move with tremendous speed and agility, and jump to great heights, displaying physical abilities far beyond those of the host organism while it was alive.

The price for these formidable abilities is that the required metabolism is so rapid and extreme that the host organism's corpse is rapidly broken down by the parasite. This is dramatically demonstrated by the rapid decomposition and disintegration of an infected life-form, which will eventually become a stunted, bloated, mobile generator and incubator for more pod infectors, which is commonly known as a carrier form.

Pod infectors and pure forms sense their surroundings using specialized tentacles, which are tipped with red ganglia. Exactly how these organs work is unknown; they may provide visual or chemical information, or perhaps a combination of both. In combat forms, these tentacles typically protrude from the chest cavity where the pod infector is housed; in pure forms, they are located on the front of the "head" segment.

The described changes occur when the Flood has still yet to overwhelm any meaningful resistance in a given locale. When this has been accomplished, available organisms will no longer be used to create purpose-built combat forms but are instead used to create a Gravemind or simply broken down and processed into biomass for a Flood hive.

Metaphysical and technological Edit

Although it would be accurate to call the Flood biological on the most basic level, as an outbreak advances, it becomes a highly powerful self-replicating swarm which will harness any means — 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.

The Flood's most notable meta-biological aspect is the logic plague, an abstract infection carried along information transmission and exchange, typically used to subvert AIs. Mechanisms used to perpetuate this infection range from subtle philosophical persuasion through mutual conversation to more predatory techniques, such as swarming AI networks with a self-replicating information infestation with the ability to convert most machine intelligences near-instantaneously. In the later days of the Forerunner-Flood war, the Flood was able to spread the logic plague through any of its biological forms or infected ancillas.

During its war with the Forerunners, after accumulating sufficient intellectual capacity and processing power in the form of several planetary Key Minds, the Flood re-gained the ability to manipulate neural physics, which served as the foundation of Precursor technology. This enabled it to control the Precursors' formerly dormant structures—most prominently star roads—and use them as weapons against the Forerunners, as well as manipulate space-time to become incompatible with Forerunner slipspace travel. On a more abstract level, the Flood's connection to neural physics caused the Forerunners to experience the very space-time around them as being increasingly hostile. Upon the development of a Gravemind, the Flood becomes capable of what can be described as telepathy among its hosts; given the Flood's origins, this ability may be related to neural physics.

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.

Developmental stages Edit

A Flood infestation goes through four distinct developmental stages:

  1. During the Feral stage, the Flood are at their simplest form: they communicate via pheromones and have the instinct to harvest enough calcium to establish a viable Gravemind.
  2. The Flood become truly dangerous during the Coordinated stage: at this point, they are controlled by the Gravemind that was created in the first stage.
  3. In the Interstellar stage, the Flood take control of space-faring technology which they use to consume the local star system. This is also the stage in which Key Minds first appear. Subsequently, they spread throughout the galaxy to infect more species.
  4. The Transgalactic stage is the penultimate stage in which the Flood utilizes all acquired means and technology to depart to uninfected galaxies to further replicate.
  5. The Universal stage is the Flood's endgame, the theoretical final stage and the ultimate goal of the infection itself. In this hypothetical future stage, the Flood will have accomplished its goal of assimilating all life in the universe. Where it would proceed from here is unknown.

Countermeasures Edit

Small-scale Edit

It is impossible to stop a Flood infection once Flood Super Cells have entered a host. There is no cure to Flood infection, although there have been many attempts. While the Forerunners could not find a cure for Flood infection, the Composer was designed to eliminate its effects by 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, 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.

Although initially used to treat those with small-scale Flood infection already present, the Composer was eventually used by the Ur-Didact to convert his Prometheans, as well as a large number of humans, into AI constructs for his war machines, in an effort to combat the Flood more effectively and without risk of infection.

Large-scale Edit

Since the Flood are a parasitic life form, they depend on other organisms for survival. Thus, the most effective way to stop a large-scale infestation is to take away their energy sources. The Forerunners first attempted this by bombing large population centers and destroying inhabited star systems via supernovae, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The last resort was the use of the Halo Array, which used neural physics to kill all sentient life in the galaxy and starved the Flood into remission. However, the Forerunners' plan included some measures to preserve the various species of sentient life and return them to their home planets, allowing the Flood's energy source in the Milky Way to persist. Unfortunately, at the same time, specimens of Flood spore cells were kept for analysis on the Halo installations, allowing the threat of the Flood in the Milky Way to be merely forestalled and not eradicated.

Environmental stimuli seem to have an effect on Flood metabolism. In the event of a Flood outbreak, Halo installations create a quarantine zone around the affected area, inside which the ambient temperature is drastically lowered. At least one Flood research facility used low temperatures to keep its specimens inactive and dormant as it studied them. At least one of the Halo installations' Control Rooms 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 weapons will burn through Flood tissue; the Forerunners incorporated directed-energy weapons into their Sentinels, 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, also reflecting the Flood's vulnerability to extremely high temperatures.

The "cure" Edit

During the Thousand Year War, humanity believed they had discovered a 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.

In reality, while humanity's efforts indeed produced the intended result and repulsed the Flood threat, it was later revealed by the Primordial that the Flood retreated by its own initiative, as humanity was not yet to be "punished" by the Flood. As such, humanity's survival was in fact due to the Flood's deliberate decision not to infect human populations, as the Precursors had determined to use the Flood to punish the Forerunners first.

After the Flood began to ravage the Forerunners' Ecumene, Forerunner scientists led by Master Builder Faber tried in vain to extract information on the cure from humans' ancestral memories, imprinted as part of their geas by the Librarian, unaware that the cure never really existed.

History Edit

The Precursors were thought to be the first sentient species to arise in the universe, predating the Progenitors, Q, and other ancient species. In time they became the most advanced and intelligent beings in the cosmos, and with this power they journeyed across space creating and seeding life, both organic and synthetic, and ensuring peace, wisdom, and prosperity for all. They discovered the spiritual concept of Living Time, the metaphysical relationship between the universe and its creatures, and developed the science of neural physics, the art of using Living Time to affect and change the cosmos in various ways. They also developed the Domain, a neural physics archive of all of their collective knowledge. The Q developed their own alternative to the Domain, a sub-dimension known as the Q Continuum, in which they resided in order to evolve beyond their physical bodies.