87.5 - 130 cm. (2 ft. 11 in. - 4 ft. 4 in.)
Small with tentacles
Piercing the skin to infect a host
The Flood Infection Form is the first 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.
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 instantly becomes a deadly foe. Any foe without shielding protection will be prone to instant death or infection, although the potential host can fight back with limited success when the Infection Form attempts to infect them. 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, though in rare cases leaving the victim conscious. 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 signals. Then, it releases the Flood Super Cell which overwrites the host DNA and causes rapid mutation.
After this is accomplished, it starts to morphologically augment the body, liquefying organs and redistributing biomass to create tentacles on the left arm from the calcium in osseous tissue. The central mass of the Infection Form will finally nest itself within the chest cavity of its host. This mutagenic process can occur within a matter of seconds. In exceedingly rare cases, such as if the Infection Form has been damaged or is incredibly old, it will go through the mutation 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.
If the infection of a particular being is not possible, that organism will be totally disregarded by Infection forms, deferring to other Flood forms the task of slaughtering it. It is likely that the Flood will use the biomass of the incompatible body for sustenance later. An Infection Form may also abandon its host if the corpse has been heavily damaged to look for a new one. Several Infection Forms, sometimes ranging from 5 to 15 at once, can attack certain victims. Infection forms appear at first glance to be quite small, but upon comparing them with the average Human, they are roughly the size of a man's torso.
Infection Forms also reanimated disabled Combat Forms if there are any nearby, following the same process as with uninfected hosts without releasing the Super Cell.
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. It is possible that this causes all members of the ORION Project to be immune. 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. This was most likely to supplement the Flood's combat capabilities as they may have been unable to muster enough proper Combat Forms to fight the enemy.
The Mgalekgolo's lack of a central nervous system and nature as an invertebrate colony consisting of multiple worms 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.
An Infection Form is capable of losing its infectious potential over time. This was experienced by Private First Class 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 to completely transform its host into a Flood Combat Form. Jenkins was only partially controlled, as there were periods of time where he could control his body and mind to some degree. For example, he was able to tell where the Flood were hiding, and saved a Marine Battalion.
Infection Forms will gather in large swarms, attacking in unison 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, weakening them enough for fellow Flood forms to kill the victim to allow the Infection Form to start the process or infect them while they are still alive.
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 effective as well, 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.
Infection Forms are spawned from bulging, sac-like pods in Flood Hives, called the Flood Growth Pods. They are only released if the pods are shot at (although for some gameplay scenes such they will pop under game programming). Unlike the Carrier Form, throwing sticky or incendiary grenades will not destroy the Infection Forms inside.
- When facing Infection Forms always remember that they are little to no threat individually and only pose a threat when in great numbers. However, if your shields are down, infection forms can be incredibly dangerous as one can take down much of your health.
- Usually if you can destroy one or two in a group they will pop and cause a chain reaction that will destroy all the others in the swarm.
- It's greatly recommended that you destroy more than one at once, but you can destroy one by one or swarm by swarm, if you think you can.
- DO NOT use a Shotgun on Infection Forms as they are so small that it would not do much damage and Shotgun ammo is far more valuable to use on the more dangerous Combat Forms and Pure Forms.
- DO NOT use an Energy Sword on Infection Forms as they are hard to hit due to their size, and the large number groups they appear in means it would take a long time to kill a whole group.
- 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, but are really a greenish color.
- In Halo: Combat Evolved and Halo 2, 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. This is because the reanimating process is not seen in Halo: Combat Evolved and only seen in dead flood forms in Halo 2.
- 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.
- 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. As with gained points, killing Infection Forms while the Flood is your ally during The Covenant will not take away points from the meta-score.
- Over the Halo series, the popping radius of the Infection Forms have dropped. In Halo: Combat Evolved, one could defeat a whole swarm by just shooting at one or two Infection Forms, causing the others to pop behind them. In Halo 3, the popping radius has been drastically decreased.
- In Halo 3, after an Infection Form successfully infects a target or is destroyed while infecting the target, small flood-like sacs are visible and eventually pop over time, although this does not cause any damage to the host.
- 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 than 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. This sound seems to be only present in Halo: Combat Evolved and is not present in the other Halo games.
- 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, like the enemies in those maps, they cannot be accessed and can only be shot at.
- 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, they will have no control over their body if an Infection Form latches onto them.
- In Halo 2, when Infection Forms latch onto hosts, it will pop up until the host's last bar of health. After the last bar of health, an Infection Form can instantly kill.
- The Infection Form can be temporarily incapacitated with high levels of pain evidenced in PFC Jenkins being able to gain control after his wrist and arm breaks when the Infection Form tries to attack its captors by sending a tentacle out of Jenkins' hand.
- The Infection Form in Halo 3 appears to have a thorn-like structure on the topside. This may be used to help it penetrate its host's body.
Halo: Combat Evolved
Halo: Combat Evolved Anniversary
Halo 2: Anniversary