Pod infector

The Flood infection form is a stage of the virulent Flood xenoform. It is the most commonly seen stage of the Flood, and is the form responsible for spreading the Flood infection.

Description
The Flood infection form might appear at first glance to be little more than a harmless and repulsive creature, but once it gains physical contact with a biological host, it becomes an instantly deadly foe. Small tentacles underneath its body penetrate the skin, tap into the victim's spinal cord, and unleash an attack on the host's nervous system via direct contact with the spine, killing the host almost instantly. Once this is accomplished, it rewrites the neural pathways of the victim's brain with its tendrils, forcing a resonant frequency match between its neural signals and the host's.

Encapsulated Flood Super Cells are then released into the host organism, transforming into full-functioning Flood cells. These cells interface with the host's cells, analyze their genetic code, and then digest them, overtaking the victim's biomass at rapid, chain-reaction speeds. This process leads to the host being overtaken by the Flood infection, allowing the infection form to use the host corpse as it sees fit, because of the horrific thought of this happening to a host it is evident that most Humans would rather commit suicide than going through the pain of infection.

After this is accomplished, the infection form burrows into the host body, moving aside the internal organs and taking up residence within the chest cavity. In order to facilitate further use of the infected organism, the Flood infection consumes the host's biomass as fuel for further sustenance and as material for further modifications of the infected life form. Common examples of these include the partially-transformed state of a combat form, whose body is host to Flood-generated structures, and the creation of a carrier form, which serves as a mobile, explosive incubator for more infection forms.

In exceedingly rare cases, such as if the infection form has been damaged or is incredibly old, it will go through this process without killing the host, leaving them at least partially aware of what is going on, but unable to move or act. This was the fate of UNSC Private Wallace A. Jenkins during the raid on the "Covenant weapons cache" on Installation 04. This aged version of the infection form must have been kept in captivity for far too long, thereby reducing its potential of complete transformation of its host into a Flood Combat Form. Jenkins survived infection, and even remained conscious, with short periods of time where he could exert control over his mutated body.

An infection form may abandon its host if the corpse has been heavily damaged, and look for a new one. If the infection form inside a combat form has been destroyed, but the combat form itself is intact enough to continue serving its purpose, loose infection forms may burrow inside the body and take the place of the one that mutated it, effectively "re-animating" the combat form. Infection forms appear at first glance to be quite small, but if compared with the size of an average human, their true size is revealed, comparable to the size of an average human upper torso.

While infection forms are essential in early stages of infecting a population, they are by no means the only way the Flood can facilitate infection. It appears that merely introducing any Flood biomatter into the body of a host will begin the infection process. However, it seems no other method of infection is able to instantaneously kill and transform a host like an infection form can.

Immunities and defenses
Energy shielding negates the effect of the infection form by causing it to explode on contact in a small, damaging cloud of gas. However, if no shield system is present on the target, the infection process begins.

At this time, the only known human to be incompatible with Flood infection forms is Staff Sergeant Avery Johnson due to his supposed contracting of Boren's Syndrome from the residual radiation of a crate full of plasma grenades on Paris IV. However, Johnson actually contracted this immunity from the augmentations he received as part of the ORION Project. According to Cortana's analysis of various Flood forms, Kig-yar and Unggoy generally lack the necessary calcium stores and biomass to be converted into combat forms and are instead used as biomass stores and as carrier forms, although Kig-yar and Unggoy combat forms have been observed during one engagement.

The Mgalekgolo's lack of a central nervous system and nature as an invertebrate colony consisting of multiple worms likely make them immune to Flood infection. The invertebrate nature of the Yanme'e, along with their hard, chitinous exoskeleton, would seem to make them immune as well.

Combat
Infection forms will gather in large swarms, attacking with their claws and tentacles. If the victim's shields are active, as explained earlier, the infection form will burst. However, the sheer force of numbers can sometimes deplete a victim's shields, allowing these Flood forms to take control of the Human/Brute/Elite body.

A single round from any weapon will burst an infection form. For this reason, automatic weapons are recommended. Dual-wielding Plasma Rifles, Brute Plasma Rifles, Spikers, or using a UNSC ballistic weapon all work well. The Battle Rifle, with its bursts of fire, can be quite effective, the Shotgun works well too when shooting at large groups closing in. Try to avoid using weapons such as M6 Handguns, or Plasma Pistols, as their rate of fire is very low. Also, try to avoid using Grenades, as they close in extremely quickly and you may suffer damage from your own grenade. In Halo: Combat Evolved, automatic weapons like the Assault Rifle and Plasma Rifle are good on them, the Needler in many ways can also be good as the needles have homing ability: be careful though, do not stick too many Needles on one at a time while they are close as if you do it will explode and effect you most likely depleting your shields. SMGs are incredibly effective for taking on swarms of infection forms with its large magazine size and impressive rate of fire. The Sentinel Beam is also extremely effective and can swiftly obliterate entire swarms of infection forms at long ranged distances.

Infection forms are spawned from bulging, sac-like pods in Flood Hives, called the Flood Growth Pods. It may be tempting to destroy these sacs, but it is actually more advisable not to, since destroying them will not kill the infection forms inside.

Trivia

 * Infection forms are carried and incubated by Carrier Forms and from Flood Growth Pods in Flood-infested areas. Try not to destroy these sacs, unless you think you can take them one by one, or swarm by swarm.
 * In Halo: The Flood, infection forms are mistakenly referred to as being colored white.
 * In Halo: Combat Evolved, if an infection form latches itself onto a Marine or one of the Covenant, instead of mutating the host, it will become irrevocably attached until the host is dead.
 * An Energy Sword will not lose energy by killing infection forms in Halo 2 and Halo 3, but this method is highly ineffective as infection forms are extremely hard to hit with an Energy Sword.
 * The Tank Form has the ability to spit out infection forms. This is demonstrated during Halo 3 for the Tank Form and Halo 2 for Gravemind.
 * Infection forms do not add points to your meta-score when the Campaign Scoring is activated in Halo 3. This is most likely because infection forms come in swarms, and could be used to rack up points with almost no effort.
 * Infection forms operate in a similar fashion to the chestbuster Xenomorph (being inside a host's chest) and the facehugger Xenomorph of the Alien franchise, the Borg Nanoprobes of the Star Trek franchise, the Headcrabs from the Half-Life series, and the Thing from the John Carpenter film of the same name.
 * The infection forms in Halo 3 float in water. Thus, they cannot infect corpses that are in water ponds because of this animation. (E.g. on the level Halo it will appear different close up then it will farther away.) However, in Halo: Combat Evolved, they sink.
 * In the book Halo: The Flood, an infection form almost infected John-117 when he turned his back, due to his energy shields being drained.
 * According to the book, Halo: The Flood, the infection forms make a "meep" sound. This can be heard in the games, although it sounds more like an ominous droning.
 * In Halo: Combat Evolved, if you shoot one infection form in a swarm, it will pop and cause a chain reaction causing all the other infection forms to pop with it in the swarm. This was less effective in Halo 2, but returned in Halo 3 (although not as powerful as in Halo: Combat Evolved).
 * Several can be seen within the Flood Observation chamber on the Halo 3 Multiplayer map Cold Storage. These Flood make Cold Storage the 6th multiplayer map with NPC enemies (Along with Construct, Guardian, Last Resort, Epitaph and Isolation which have Sentinels, Constructors, seagulls or pterodactyl-like birds flying around). However, you cannot get to them.
 * In Halo: Combat Evolved, the infection forms have a pulsating glow in the dark.
 * The infection form was featured in Halo Wars as a non-playable unit for the Flood. They have the ability to crawl over walls, being able to reach ground no other forces can reach and transform most infantry into Combat Forms.
 * On the Halo 3 level Halo, when in the Warthog, Mongoose, Chopper, etc., if you drive into the infection forms you simply drive over them, possibly flipping your vehicle. Performing this action kills all other Flood forms including the carrier forms, although the latter explode violently on contact.
 * In Halo: Combat Evolved, if players listen close enough to the droning sounds emitted by the Infection Forms, it can be noted that the sounds are actually the same sounds used for "Pfhor Fighters" death screams and occasional random chattering from the Marathon franchise. However, the sounds have a slightly higher pitch, as well as oozing sounds, which make the reuse of the sound effect from Marathon almost completely unnoticeable.
 * In Halo 3, despite their seemingly weak form, they can actually move certain objects that are bigger than they are. This can be observed in the crashed ship on the Halo 3 level Floodgate and in the level Cortana.
 * In Halo: Combat Evolved it is possible to "stick" an infection form with a plasma grenade.
 * In Halo Wars, when Sergeant Forge shoots the Flood infection forms that are attacking Professor Anders, they do not pop like all other infection forms. Instead, they simply fall lifelessly to the ground.
 * In Halo PC, if the player uses cheat_bump_possession to take over a Marine or a Grunt, they will have no control over their body if an infection form latches onto them.