Pod infector

The Flood 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 at first glance to be repulsive but harmless, but once it gains physical contact with a biological host it becomes very dangerous. Small tentacles underneath its "body" penetrate the skin, tap into the spinal cord, 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 left hand from the calcium obtained and finally nest itself within the chest cavity of its host. After liquifying the insides, it will begin to liquify the outside body to harden, making a very hard "carapace". 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 necessary calcium needed to mutate. However, though too small to be useful as Combat Forms, the latter two are instead transformed into Carrier Forms.

Infection Forms in Halo 2 were rushed in the making, and you will probably notice that they move strangely and could be considered that they skim the ground while moving.

Trivia

 * Infection Forms are created by Carrier Forms and from Flood Growth Pods in Flood-infested areas. Try not to burst these sacs unless you think you can take them.
 * 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 but their death noise is different form Halo CE than just a popping noise.
 * 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/vomit out Infection Forms.
 * Infection Forms do not add to your meta-score when Campaign Scoring is on.
 * Infection Forms operate in a similar fashion to the Facehugger Xenomorph from the Alien franchise.
 * In Halo: Combat evolved, when the player shines their flashlight on an Infection Form, the Infection Form will scurry out of the light.
 * The Infection Forms in Halo 3 float in water, and they cannot infect corpses that are in water ponds because of this animation.
 * In Halo: Combat Evolved, Infection Forms can latch into Shade turrets, causing the stationary gun to fire off-target. Killing the Infection Forms causes the Shade to fire properly again.
 * In all Halo games but Halo 3, the Infection Forms are the first Flood form seen.
 * In Halo 3's Theater mode, if you detach the camera and go to a swarm eg. (on the level Halo) it'll look different close up then it will farther away.
 * In the Halo 3 Campaign, if you shoot the pulsating Flood sacs on most Flood levels; Infection Forms will pop out. However, Isolation is an exception because it is a multiplayer level and the sacs have no infection forms in them. Instead, they merely scream and develop a rip similar to the rip seen on normal sacs.
 * If an Infection Form leaps on an unshielded player in Halo CE and Halo 3, the player will lose some health and the Infection Form will leap off and attempt to get back on.
 * If an Infection Form leaps on an unshielded player in Halo 2, the player will die instantly.
 * When a flood combat form is killed but not dismembered, these can leap onto the dead body and bring it back to life.