Flood pure form

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. Because they employ a calcium-based exoskeleton for protection, these forms are significantly more resilient than the conventional Flood forms encountered in the first two Halo games. Genetically, they are composed of the same Flood "super-cell" which infects and transforms all other victims of the Parasite, but are made up entirely of recycled bones, muscles and other biomass collected by the Flood, rather than temporarily reanimated host bodies. This biological configuration affords significant advantages. Many weapons such as the shotgun, battle rifle, carbine and assault rifle, which easily dismember other Flood forms, prove largely ineffective against these calcium-hardened opponents. However, once you do kill them, they cannot reanimate, much like non-Flood enemies. There are three types of pure forms: Stalker Form, Ranged Form, and Tank Form. Any one Form can mutate between these sub-forms at will, with the Stalker seeming to be the "base parasite" that the other two forms mutate from. (i.e., if a Tank wanted to become a Ranged Form, it would first have to turn into the Stalker, but there have been occasions when a Tank mutates into a Ranged).

Weaponry that HAS proven effective against these Flood types includes the Flamethrower, the Energy Sword, and the Needler. The Plasma Rifle can be effective at very long ranges, even on Legendary. For Lower Difficulties melees with Brute Weapons have considerable damage often only needing one or two hits. Incendiary Grenades are one of the most effective weapons against this Flood Form, taking down any Flood form with one hit and incurring subsequent damage to any nearby Flood. As a general rule, heat-based weaponry (that which utilizes any form of plasma or flame), or percussive weaponry (Gravity Hammer, Rocket Launcher, Brute Shot or any kind of grenade), are the easiest ways to dispatch Pure Forms.

Stalker Form
Stalker forms are extremely agile and can jump large distances. They tend to crawl on ceilings and walls before leaping at an opposing enemy. Generally speaking, the Stalker Form, even on the Normal and Heroic settings of the game, are too fast to be aimed at for any significant period of time. However, should one drop down from the ceiling, the best policy is to melee it, as a melee is a one hit kill. Don't waste ammo trying to shoot it, they are very resistant to bullets. They attack far more rarely than the other two forms. If you leave one alone for some time, it will either become a Tank or Ranged form.

Ranged Form
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. These are slightly more tricky to kill due mainly to the fact that they can have you pinned down in a position for a very long time. If they are left alone for some time, they will change back into a Stalker form, get a better angle, and become a Ranged form again. The general consensus is, however, that if you have access to a Needler at a medium distance and fire a volley, they go down easily enough. Plasma Rifles tend to work just as well at closer distances. Also note that this Pure Form may shield its self by curling up, it is then almost invulnerable (the Flamethrower, Energy Sword, Gravity Hammer, general explosives, and sustained Spiker fire will still kill it). A Brute Shot may be the best way to knock a Pure Ranged Form off of a wall by means of one or two shots. Please note that when changing form it is easiest to kill, seeing how it can't move while changing and it seems to have the health of the Stalker Form.

Tank Form
Tank Forms are large and easily spotted. Unlike the other Flood Pure Forms, they can not climb on walls and ceilings. These Flood can take a tremendous amount of damage and can kill you in one hit in higher difficulties. They also occasionally spew Infection Forms from their 'mouths'. The Energy Sword is the only weapon that can kill one of these flood forms in one hit. A Flamethrower can also kill a Tank Form with only a small amount of flame used. A Plasma Rifle has also proven effective to a certain degree. Also, the melee of a Brute Shot can be very effective, especially if used in a "hit and run" style, with the player rushing up, meleeing, and then retreating (2 hits are needed on Legendary, or a shot and a melee). This tactic is very effective when Tank Forms are supporting a large number of Combat Forms, as it frees up much needed ammo for fighting other Flood troops. Also it is helpful as the Brute Shot melee has infinite "ammo," so one can essentially take down a whole horde with clever melee tactics. However, please note that when changing form it is easiest to kill, seeing how it can't move while changing and it seems to have the health of the Stalker Form. When use of a Shotgun is necessitated, get close and aim for the target's "mouth", however the Pure form will occasionally attempt to cover its "mouth" with its left arm while charging, making headshots difficult.

Trivia

 * The Energy Sword is the only weapon that can kill a Tank Form with one hit. However, on lower difficulties, it is possible to kill a tank form with one melee hit from a Brute Shot.
 * It is theorized that the Flood Juggernaut, a Flood Form cut from Halo 2, was an early render of the Halo 3 Pure Form. The Drinol, another deleted species from Halo 2, may have served as the inspiration for the Tank Form This is further proven by a piece of concept art on the Essentials disc of the collector's edition of Halo 3 that appears to be three Flood Juggernauts engaging several Marines and the Master Chief in combat.
 * If a Pure Form is stuck by either a Plasma Grenade or Spike Grenade while mutating, the grenade will be "absorbed" and no explosion will occur.
 * The Needler is extremely effective against Ranged Pure Forms, as it is the usual 7 projectiles to kill it. This is most effective on Legendary, and on the level Cortana.
 * If you kill a Pure Form in the middle of changing, it may appear with strange deformities e.g. if a Ranged form is transforming to a Stalker and you kill it, depending on when you kill it, it can appear as a Ranged form with stalker feet or, more often, a stalker with over sized feet, etc.
 * The Flood Pure Forms seem less resistant to fire. When a Combat Form is lit on fire, it can usually charge towards you. Pure forms fall to fire in seconds.
 * With the IWHBYD Skull turned on sometimes if you are killed by a Tank Form the Gravemind will laugh using the Tank Form.
 * The Tank form has sometimes been mistaken for a Hunter form. However, the idea of a Hunter form is made impossible by the fact that the Hunter has no central nervous system, only a collection of semi-sentient worms, and is therefore uninfected. The Hunter also doesn't have bones, and therefore would be unable to provide enough calcium for a Flood Infection to sustain itself over a long period of time.
 * Ranged Pure Forms are incredibly resilient against munitions and plasma based weaponry, but a single melee attack will render them dead, also particularly effective is the Flamethrower
 * A Tank Form is featured on the map 'Cold Storage' in a cryo-container.