Infection Form

The Infection Form is the first stage of the Flood. It is considered to be the organism's "natural" state.

Overview
The Flood Infection form appears harmless, yet repulsive, at first glance, but once it gains physical contact with a biological host it becomes very dangerous. Small tentacles underneath its "body" penetrate the skin and unleash an attack on the host's nervous system via direct contact with the spinal cord. Once this is accomplished, it rewrites the neural pathways of the victim's brain with it's tendrils, forcing a resonant frequency match between its neural signals and the host's. Then it releases the Flood Super Cell which overwrites the host DNA and causes rapid mutation. The creature being infected during this process is still alive but has no control during this process. After this is accomplished, it starts to edge its way into the body, liquefying the organs to create the tentacles on the right hand from the calcium obtained and finally nest itself within the chest cavity of its host. If a match cannot be obtained, the host will be left alone by the Infection form, but it is most likely that the Flood will kill it (if it is not already dead) and save the host's body for later sustenance.

At this time, the only known human subject to be incompatible with Flood Infection Forms is Sergeant Johnson, due to his supposed contracting of Boren's Syndrome from the residual radiation of a crate full of Plasma Grenades. There are theories on other species immunities to the flood, such as the Hunter's lack of a central nervous system and nature as a conglomeration of multiple eel-like creatures and Jackals and Grunts for lacking neccesary calcium needed to mutate.

Combat
Infection Forms are extremely fragile, and easy to defeat by themselves. Unfortunately, they are rarely encountered alone. Dozens of Infection Forms usually follow Combat Forms and Carrier Forms into battle, infecting new victims while they are wounded or distracted in fighting. The infection forms can climb walls and ceilings, and are capable of jumping several meters (6.6 feet - 9.8 feet) by propelling itself with its tentacles.

The easiest weapons to use to destroy them are Assault Rifles, Shotguns, SMGs, and Grenades, as they usually move in swarms and one burst from these weapons can eliminate most of the swarm. If the victim does not have a shield engaged, the forms will latch on to them and attempt infection. When faced by mass waves of Infection Forms, the Assault Rifle works due to it's very quick rate of fire. The Forerunners developed the Sentinel Beam to destroy Flood, and it should be used when available. It is accessible in Halo 2 and Halo 3.

In Halo 2, Infection Forms gained the ability to revive dead, with the exception of dismembered Combat Forms, by burrowing into them. Also, the Infection Forms can destroy Sentinels if the former leaps onto the latter. If the player inflicts enough damage to the prone Combat Form it will disintegrate completely and become useless (Grenades/explosives are most effective, Energy Swords and the Brute Shot's blade.). When a Combat Form is dismembered but not killed, it will automatically die after a couple seconds and an infection form will sometimes spawn out of its chest cavity where it once resided in the host's body.

In Halo 3, the Infection Form is capable of infecting Marines, Elites, and Brutes, rapidly mutating their bodies immediately after they come into contact with their hosts, transforming them into mindless Combat Forms. If the victim has an energy shield they will burst upon coming in contact with it, only when the shield is taken out can they latch on. The experience is done entirely while the victim is alive and presumed to be extremely painful, though they will also mutate dead bodies. If you have good aim, you can shoot off the Infection Form before the mutation even begins. They also have the new ability to overturn vehicles like the warthog when grouped in large numbers.

Spoilers
On Halo CE, they first appear on the level 343 Guilty Spark until the game ends, except on Two Betrayals

Later, on Halo 2, they first appear on The Oracle, first on a sealed storage chamber at the very start of the level, and throught the whole entire level.

They do not appear until the level Sacred Icon, and then, Quarantine Zone

On Halo 3, they appear on the level Floodgate, on board of a crashed Covenant Cruiser

Later, when you deactivate the last tower on The Covenant, they arrive on board of High Charity, to the Ark. Then they appear on the level Cortana and Halo

Trivia
A Marine is infected by the Flood (Double-click for larger video) N-bCMhBINiA
 * Infection forms are created by Carrier Forms and from sac-like growths in flood infested areas.
 * In Halo 2, the sound files for the Infection Forms from Halo: Combat Evolved were remade, but in Halo 3 they were changed back to the original ones from Halo 1.
 * The Infection Forms sound like a distorted human moan.
 * The Infection Form has appeared in two Action Figure sets: the Halo 1 Flood Carrier Form Action Figure and the Halo 2 s8 Master Chief v4 Action Figure. The differences between the two versions were significant, namely the first Infection Form featured a detachable membrane that revealed the internal organs of the creature.
 * In Halo: Combat Evolved, the Infection Forms glow in the dark in an on-off fashion.
 * An Energy Sword will not lose energy by killing infection forms.
 * The Tank Form has the ability to spit out Infection Forms.
 * Infection forms do not add to your meta-score when campaign scoring is on.