The Flood

 The Flood are a highly virulent and deadly parasitic race, and can quickly incapacitate and infect different life forms, although the infected subject must have sufficient body mass and some level of sentience to sustain the infection forms, such as a Human or Sangheili.

Biology
The Flood appears in four primary forms: Infection, Combat, Carrier and Pure forms. The Flood will consume all sentient life it comes into contact with, and can mutate its hosts into various forms to help it do so.

There are three stages in the evolution of a flood colony:
 * 1) Feral: This is the first stage of the evolution: the species works as individuals and will attack anything they see – even each other.
 * 2) Colonial: This is the second stage of the evolution: the Flood first start showing signs of ethological behavior. It is at this stage that they form a hive-mind.
 * 3) Civilizational: This is the final stage of the evolution: this stage is triggered when the colony reach a critical biomass; when they first start making a Gravemind. This is the most dangerous of the stages: it is the point when all Flood forms are united and actually start thinking like a hive-mind, and reach human-level intelligence: they can coordinate attacks on specific locations or targets and start utilizing technology.

Infection Forms
The infection forms are the most common type of "Pure" Flood spawn and are the truly parasitic members of the Flood. They have a very strange anatomy: they are small, tentacled creatures which have their own defined biological framework, and therefore are not dependent upon a host. Individually they are weak, parasitic creatures which will try frantically to take over a host; together, they can easily overwhelm a platoon of fully armed marines.

An infection form will seek out any sentient life of capable biomass and calcium deposits to sustain itself, and proceed to attempt to use the creature as a host. The infections forms 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 through mutagenic spores 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. These Combat forms are the frontline troops of the Flood: the species the Combat forms originate from can vary. A combat form can only be developed when there are sufficient levels of calcium in the host: if not, the host will not generate a combat form but instead will generate a carrier form.

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 Elites, 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 forms are sometimes able to use the energy shielding technology with their remaining 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. The combat forms see through the small feelers and tentacles sprouting out of where the head used to be. These are the tentacles of the Flood Infector that resides within the host.

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. Decapitating a combat form will do absolutely nothing, as the head is not utilized by the Infector and simply pushed aside.

Carrier Forms
An infected host can also mutate into a Carrier form. Carriers grow large sacs which incubate several infection forms. When a carrier is in close proximity to any number of suitable hosts or when it takes enough  damage, it triggers some sort of biochemical reaction, causing the sacs to explode and spread the infection forms in order to infect more hosts.

Carriers are slow and walk with a comical waddle, flailing their tentacles as they stumble towards the enemy. The explosion is quite forceful and is capable of doing some damage to anything caught within the blast, though not enough to injure an Elite or SPARTAN super-soldier. This is their only offensive mechanism and portrays them as suicide bombers. It is unknown what the criteria are for a host to become a carrier form. All answers are speculative at best.

Pure Forms
Pure Forms, also known as Morpher Forms, are highly advanced Flood forms, first seen late into the Human-Covenant War. Calcium from the bones of the Flood's victims makes up the structure of the Pure forms. Through some unknown means, the Pure forms have the ability to alter their structure, and possibly their entire genetic structure, as a response to different combat stimulus.

Stalker Form
Stalkers are the basic Pure forms. They are incredibly agile, and vaguely resemble an ant. Their head is the most common trait of all Pure forms, with a pair of thick tentacles framing a mouth full of Flood sensor antennae, while their abdomen, which is possibly made up of a torso of a human or similar creature, stores calcium for transformations. They have four limbs, the rear pair of which end in splayed feet used for walking, while the front pair end in scissor-like claws used in combat. However, the Stalker is very ineffectual in combat, and prefers to transform into one of the other Pure forms to fight. The Stalker moves in a spider-like fashion, climbing walls and ceilings, and is capable of extraordinary leaps. They often drop from ceilings to ambush whatever is walking underneath.

Ranged Form
Ranged forms are tripedal Pure forms. These forms are turret-like in appearance and are quite tall. They are often found hanging from walls or ceilings, from which they fire sharp spikes at enemies. The nature of these spikes is assumed to be that of battlefield detritus and shrapnel, but could also possibly be that of excess calcium converted into fragmented shards. In any event, these shards are the main identifying feature of this form, standing in multiple, somewhat organized rows up the front side of the creature's body. Physically weak, they are highly susceptible to damage from the front. To counter this, they curl into a ball to protect their weaker spots when attacked, and are able to transform into the Tank Form should an enemy get too close.

Tank Form
The Tank form is a bipedal Flood Pure form, with large, asymmetrical arms formed from the majority of the Pure form's calcium structure. Physically the strongest and toughest Pure form, the Tank Forms are very imposing, standing head and shoulders above average humans. They use their massive strength and hardened forearms to tear through their enemies, and are capable of surviving what would kill pretty much anything else. They can generate Infection forms, regurgitating them out of their "mouths" during lulls in combat. Tank forms are possibly used as a means of mobile communication for the local Gravemind, although there is only one documented instance of this. Tank forms are all but invincible when it comes to ballistic and energy ranged weaponry save for shots delivered to the face centre where its fan-like tentacles lie. However, the Tank form may attempt to shield its face with its arms when fired upon.