Flood Forms

Flood Spores are the most simple Flood organism, facilitating the creation of infection forms out of available biological material to advance infestation. These are small airborne contaminants, capable of terraforming the surrounding environment.

The Infection Form is the second most basic and simple form of the Flood, and is larger only than the Flood Spores, and thus the easiest to defeat. It is believed to be the most essential form to the flood, as it is the Flood's most common method of infecting and assimilating other species, as these infections attack in dozens or even hundreds at a time. Interestingly, the infection forms pop like balloons when you destroy them in Halo: CE.
Strangely enough, in Halo: Combat Evolved, the Infection Form doesn't infect enemies; it just kills them.

After an Infection Form has successfully assimilated a sentient creature with sufficient biomass and combat potential, it will neurologically and physiologically mutate the host, twisting it into a Combat Form. Combat Forms oftentimes maintain the general profile of the original host organism, and possess the original clothing and armor they were wearing before infection. Infection forms have been seen mutating Humans, Sangheili, Jiralhanae, Unggoy, and Kig-Yar into combat forms. Mgalekgolo cannot be infected due to their lack of sufficient calcium and a central nervous system. Also, Yanme'e cannot be infected either, since they also lack protein, and a basic skeleton in which the Flood craft their biomass into.
When a Combat form has served its usefulness, the infection form may dislodge itself from the body and look for a new host, leaving the body for use as spare biomass. Alternatively, the Combat form could be made into a Carrier form.

The exact origin of Carrier Forms is unknown, although they are theorized to be former combat forms that are too damaged or too aged to be useful for future combat. The purpose of a Carrier Form is to nurture and disseminate Infection forms upon attack or proximity to a host. It is also rumored that flood carrier forms may just be grunts transformed by the flood. Although the legs are rather stumpy, much like a grunt's, the inflated "body mass" that houses the infection forms is far too big to actually be a grunt's, considering that if grunts skin was stretched out, it would not cover nearly enough space to be the size of the actual form. Although, it could just be the infection's "skin" stretched over the contents.
This type of Flood is first featured in Halo 3. As implied by their name, Pure Forms are not the product of infecting a biological host, but are instead composed entirely of Flood biomass. They are a new stronger strain of Flood that develops once a coordinated stage has secured sufficient resources, namely in the form of calcium from host species. The Flood make use of those reservoirs of calcium to develop an adaptable framework upon which the Flood Super Cell is attached, which creates a fearsome biological war machine. Initially, this form's primary function is to aid Infection Forms by securing more biomass in the form of hosts, as well as enough complex neurological systems to strengthen the local Gravemind. This duty overlaps with the Combat Form, but once enough Pure Forms have developed, the Combat Forms are relegated to defensive roles, or adding to the biomass and calcium reservoirs. Flood Stalker forms, Tank forms, and Ranged forms are Flood Pure forms that appear in Halo 3. Flood Transport forms, Infector forms, and Stealth forms are Pure forms that were cut from the final version of Halo 3. Flood Bomber forms, Swarm forms, and Thrasher forms could possibly be Pure forms, but this is unlikely. The Pure Forms also bear a lot of resemblance to the Mgalekgolo and use the same animations for melee's.

Stalker forms are extremely agile and can jump over large distances. They tend to crawl on ceilings and walls before leaping at an opposing enemy to attack. A stalker form usually turns into a tank form when a target gets close to it, and a ranged form if you're shooting at it and a wall is nearby. Ranged forms mainly turn back into stalkers before transforming into tank forms.

Ranged Forms can usually be found clinging to ceilings or walls, though unlike the Stalker Form, they rarely move from a given spot. They can fire sharp projectiles at you, without the need of a weapon. The ranged form is susceptible to the Covenant Needler. It is especially weak against plasma, spike, and firebomb grenades, though they don't usually have a long range. The ranged form can also fold itself while remaining on a wall; when folded, it is much harder to kill. Also the spikes may look familiar because they are similar to spikes from a Brute Spiker. Sometimes, when on Legendary difficulty, the Ranged Form will drop Spike Grenades, and like Spike Grenades, their projectiles do no damage to Deployable Cover, making it an ideal defense against them.

Tank Forms are large and easily spotted. Unlike the other Flood Pure Forms, they cannot climb on walls and ceilings. These Flood can take a tremendous amount of damage. They also occasionally spew Infection Forms from their 'mouths'. Tank forms may be mistaken for a (nonexistent) Hunter form, mainly because a tank form has one over sized hand (equivalent to a Hunter's shield), a small arm (equivalent to where a Hunter's gun would be), its roar which sounds similar to that of a Hunter, and even more similar body mass. It is however, impossible for a Hunter to be infected (despite these similarities) due to the fact that Hunters are a colony of invertebrate worms lacking bones (i.e. calcium) and with insufficient biomass per worm for an infection form to modify. There is speculation that it is a revised form of the Flood Juggernaut.

Prophet Forms are rare products of infestation. Because of this, many mechanisms and details of Prophet Forms are unknown, although it appears that the Prophet Form is directly infected and incorporated into one of the many tentacles of a Gravemind form, and fully retain the original voice and full intelligence of the former Prophet.
The Prophet Form is like the Combat Form in the physiological distinctions it shares with the original organism before infection: Prophet Forms can easily be compared to Prophets by their heads and, to a lesser extent, their chests, merely having a central body of the Gravemind tentacle. Only one infestation process of the Prophet form is seen, but yet to be completed (High Prophet of Truth in Halo 3).

A very rare form of flood is the Brain Form (a.k.a Proto-Gravemind). This form is unlike most other Flood forms since it lacks any sort of mobility or defense, and is passive. These are, theoretically, the beginning stages of a Gravemind form. It is believed to be the product of syntactical fusion of several combat forms, merging its entire subordinate forms into a massive, bloated spherical organism with spindle-like outgrowths that adhere to the surrounding environment.

Proto-Gravemind forms are unique in that the consciousness of the original host organisms are still vaguely maintained, greatly unlike most combat forms, allowing for slight manifestations of mental resistance to Flood assimilation and mental communication with other humans. However, the Flood is able to, with high intuitive perception, sift through the individual memories of the original hosts, deleting erroneous ones while seeking memories that are of great importance to the overall Flood species. Once such a memory is found, it is incorporated into the general consciousness of the Flood species.

The only known human example currently is Captain Jacob Keyes, who was originally assimilated into a Combat Form, and was then merged with the Proto-Gravemind on the bridge of the CCS-class battle cruiser, the Truth and Reconciliation. He was transformed into a Proto-Gravemind as he had superior knowledge about the Pillar of Autumn and Cortana. It was destroyed when a squad of Special Operations Elites infiltrated the bridge after John-117 had escaped the cruiser aboard a Covenant Banshee.

The Gravemind is the leader intelligence of the Flood. There can only be one at a time, although if one is killed another may form, though it is unknown if the dead Graveminds knowledge is passed on to the new one, but the statement above supports this. Another thing that may happen is that the Gravemind is the same from before in a different body. The Gravemind is the assumed highest state of transformation that a Flood form can achieve. It is believed that once a "critical mass" of Flood forms has been synthetically fused, similar to the Brain Form, but on a larger scale (and very likely the next stage of a brain form), a Gravemind form is created. It assumes a centralized sentient intelligence that is believed to lead the general Flood species itself through unknown means, possibly telepathic. It also appears to hold all of the Flood's memories, even those of individual organisms, and of the previous Gravemind as well, through extraordinary and enigmatic "genetic" means. While it is capable of directing the overall Flood species, it also possesses micro-managing capabilities for small encounters.

The Gravemind is the personification of Flood psychology, ravenously seeking to assimilate all sentient life in the galaxy, and is of unusual intelligence: of inventive plans for Flood propagation and serpentine delusions and betrayals, capable of manipulating others to do its bidding. It appears that only one is present at a time[21]: one existed over 100,000 years ago, but was destroyed by the first firing of the Halos. Another was recreated afterward, and settled in the depths of Installation 05: it showed remarkable understanding of Forerunner technology, even going so far as to capture the resident Monitor, 2401 Penitent Tangent, manipulating the installation's teleportation system, and understanding the purpose and intricacies of the Halo installations. The Gravemind is also remarkably long-lived, surviving for a millennium.
Physically, it somewhat resembles an engorged Venus Fly Trap: it is a "massive, tentacled...entity"[22] with a gigantic, fleshy neck and indistinct head, prominently featuring a massive mouth that speaks in low, gravely tones. It possesses massive tentacles of extraordinary length and strength, capable of stretching from the dark interior of Installation 05 and believed to be extending hundreds of kilometers to above the surface of the Halo. It is also believed that a Gravemind can weigh several thousand tons.

The Flood Swarms are the first seen flying incarnation of the Flood. They travel in large airborne packs and emit barb-like projectiles to puncture enemies similar to the Ranged Pure Form.

Another airborne parasite, these large floating creatures move slowly and have the ability to drop Flood Growth Pods full of Infection forms on enemies. This appears to be their only form of attack, making them very vulnerable when attacked from a long distance. The Bomber's role of dropping Infection forms on enemies is similar to the Carrier Form but instead of blowing itself up at close range, it drops pods filled with Infection forms from the air, thus making it more useful at spreading infections than the Carrier Form in most situations.

The Flood Thrasher Form is a Flood form, similar in purpose and behaviour to the Flood Tank Form, but larger and somewhat resembles a Thorn Beast. It is intended to quickly close with enemy ground units, using a devastating melee attack to deal significant damage. So far, they have only been witnessed as a part of the Flood forces on the Flood-controlled Shield World. These have only been seen in Halo Wars.