Infection Form

The Infection Form is the first stage of the Flood. It is considered to be the organism's "natural" state. It is also referred to as the "spore form" (which probably is an incorrect name as the smallest form floating around in the air would be called a spore).

Background
The Flood Infection form appears harmless, if repulsive, at first glance, but once it gains physical contact with a biological host it becomes very dangerous. Small tendrils underneath its "body" penetrate the skin and unleash an attack on the host's nervous system via direct contact with the spinal cord.


 * First, it paralyzes the host's body ensuring the host does not struggle.
 * 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 Flood DNA which overwrites the host DNA and causes rapid mutation. (Please note that the Creature being infected during this process is still alive, there are theories that say "The Conscious of the being is slowly pushed into the back of it's mind, unable to move his/her body and can even see what is happening." Almost like Private Jenkins except they're unable to control their body, they only sit back and watch)
 * After this is accomplished, it starts to edge its way into the body, slowly moving organs aside so it can 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 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 the Jackals which have too frail bodies.

Graphical downgrade
In Halo CE, the Infection Forms were texture rich, however with the release of Halo 2, it appears that their texture has been downgraded, appearing as a low resolution blob of colors. This has changed in Halo 3, where the Infection Forms are now textured as dull shades of grey, losing their greenish hue in the process. The red protrusions are now more visible.

Combat
Infection Forms are extremely fragile, and easy to defeat by themselves, even exploding when near loud noises. 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. Jumping or other aggressive actions can get them to detach. 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 melee, and the Plasma Rifle are also found to be effective, as unlike the Energy Sword the melee function doesn't have a limited usage amount. However, the accuracy of a melee attack on an infection form is much lower in Halo 2; the infection forms may have begun to learn that a direct jump at the Master Chief's chest is a low-percentage strategy, and have adopted craftier attack patterns. If they latch on to an unshielded entity (even standard Sentinels), death is near-instantaneous.

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. When the Flood temporarily ally themselves with you, Infection Forms ignore the Chief and Arbiter and home in on all Covenant Loyalist forces in the vicinity. If they leap on to an unshielded player character, they will deal some damage, leap off, and then attempt to get back on, much like in Halo: Combat Evolved.


 * A video of the Flood Infection Form mutating a Marine

Trivia

 * 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.
 * In Halo 2, 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. This does not occur in Halo 3, and instead the Combat Form will charge toward you.
 * An Energy Sword will not lose energy by killing infection forms.
 * In Halo 1, Infection Forms appear in greater numbers, but chain react when destroyed, popping others nearby. This factor was removed in Halo 2 and their numbers were lowered to compensate. In Halo 3, they once more chain react and appear in large numbers.
 * In Halo 1, if you shine the flashlight on Infection Form flood, they will get out of the way of the light. It is possible to actually shine the light on them in order to make a safe path. However, it is unwise to rely heavily on such a tactic.
 * In Halo 3 when you pause the film in saved films and move away from a Infection Form, it will change position. When you are far away of the infection form it legs will stand different than when you look at them close.
 * The sacs on the walls in various levels of Halo 3 contain large numbers of Infection Forms, which are released if you melee or shoot them.
 * The Tank Form spits out Infection Forms.
 * Infection forms do not add to your meta-score when campaign scoring is on.