A Main Character

Though they appear small and relatively harmless, a single Flood Infection Form can be responsible for the extinction of an entire species. Their primary function is to gather biomass, and they can achieve this in a variety of ways.  Any sentient being without shielding or other suitable protection are the most vulnerable. There have been reports of some being able to resist infection (see service records of PVT Wallace Jenkins, CPT Jacob Keyes), and others that are entirely immune (SSG Avery Johnson).

Infection

The process of infection begins with host contact. Tentacles protruding from its body penetrate the skin, and attempt direct communication with the host's spinal cord. Once communication is established, a potent attack of the nervous system is commenced. In the vast majority of cases, this kills the host almost instantly. It then rewrites the neural pathways of the victim's brain, forcing a resonant frequency match between the host neural signals and its own. Then, it releases the Flood Super Cell which overwrites the host DNA and causes rapid mutation.

Once the internal infection process is complete, it begins to morphologically augment the body liquefying organs and redistributing biomass to create large claw-like appendages on the arm from the calcium in osseous tissue. The central mass of the Infection Form will then nest itself within the chest cavity of its host, completing infection of the host. Though complex, this mutagenic process takes seconds. In exceedingly rare cases, such as if the Infection Form has been damaged or is excessively aged, it can complete infection without killing the host. This leaves the host at least partially conscious, but with little to no control over body and mind.

If the infection of a particular sentient being is not possible, that organism will be completely disregarded by Infection Forms. The incompatible body will then be killed and used as biomass as the Flood spreads. An Infection Form may also abandon its host if the body has been heavily damaged.

Infection forms can be dispatched with a single shot from any weapon (heat is most effective), but can easily overwhelm in large numbers.

Infection Forms can reanimate disabled Combat Forms and infect deceased bodies. Burn these potential hosts whenever possible.

Immunities and Defenses

Energy shielding is most effective in defense against Infection Forms. They simply explode when they come into contact. This splattering action does drain energy shielding by a small margin, so proper care must be taken to avoid a potentially fatal drop in shield strength.

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. This was thought to be caused by the residual detonation radiation of a crate full of plasma grenades on Paris IV. However, his immunity actually developed 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.

Deterioration

An Infection Form is capable of losing its infectious potential over time. This was experienced by Private First Class Wallace 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.

Combat

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, Jiralhanae 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 plasma or incendiary grenades will not destroy the Infection Forms inside.

Tactics

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.

They can often times be destroyed by simply walking into them, but this is a risky move because they can chip off a small but considerable amount of shield. Not recommended for large swarms.

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.

Spending ammunition, especially Shotgun rounds, on Infection Forms is not reccomended and should only be considered in extreme emergencies or if replacement ammo is around. They are often too little a threat to justify trying to kill, and their small size can lead to wasted shots. While the Shotgun can cause a chain reaction and wipe out an entire group if clustered together, the 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.