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".

Background
The Flood Infection form is appears harmless, if repulsive, at first glance, but once it gains physical contact with a biological host it becomes highly 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 the rapid mutation.
 * 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 contracting 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.

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 jump 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, but it is only accessible in Halo 2.

In Halo 2, Infection Forms gained the ability to revive dead but not 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.

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 spawn out of its chest cavity where it once resided in the host's body.
 * 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 1, if you shine the light from an assault rifle 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.