Pod infector

The Flood Infection Form also known as a Flood Spore, the first stage of the virulent Flood xenoform and is the most commonly seen stage of the Flood and the form that infects lifeforms to spread the Flood infection.

Description
The Flood Infection Form appears at first glance to be repulsive yet harmless, but once it gains physical contact with a biological host, it becomes an instantly deadly foe. Small tentacles underneath its "body" penetrate the skin, tap into the victim's spinal cord, and unleash an attack on the host's nervous system via direct contact with the spinal cord, thus rendering the host's legs useless, so that he/she won't be able to run. 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 liquefying the insides, it will begin to liquefy 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, although it is likely that the Flood will eliminate it (if it is not already dead) and save the host's body for later sustenance.

Immunities/Defenses
Energy Shield Systems negate the effect of the infection form by causing it to explode on contact in a small, damaging cloud of gas; however, if there is no shield system present on the target, the infection process begins.

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. According to a UNSC analysis of various Flood forms, the Jackals, lacking necessary calcium stores and biomass to mutate into a Combat Form, are instead transformed into Carrier Forms, which carry even more Infection Forms.

There are theories on other species immunities to the Flood, such as the Hunter's lack of a central nervous system and nature as an invertebrate colony consisting of multiple Lekgolo Worms. The invertebrate nature of Drones, along with their hard, chitin exoskeleton would seem to make them immune as well. Also, because of his contraction of Boren's Syndrome, Sergeant Avery Johnson seems to be immune to flood infection; however, it is speculated that could be because he was in the SPARTAN I program.

Trivia

 * Infection Forms are carried and supported by Carrier Forms and from Flood Growth Pods in Flood-infested areas. Try not to destroy these sacs, unless you think you can take them one by one, or swarm by swarm.
 * Infection Forms are capable of destroying Sentinels in Halo 2.
 * 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.
 * An Energy Sword will not lose energy by killing Infection Forms(this method is highly ineffective as infection forms are extremely hard to hit with an energy sword).
 * The Tank Form and Gravemind have the ability to spit/vomit out Infection Forms.
 * Infection Forms do not add points to your meta-score when the Campaign Scoring is activated.
 * Infection Forms operate in a similar fashion to the Facehugger Xenomorph from the Alien franchise.
 * The Infection Forms in Halo 3 float in water. Thus, they cannot infect corpses that are in water ponds because of this animation. (eg. on the level Halo it'll look different close up then it will farther away.)However, in Halo 1 they sink.
 * In the book Halo: The Flood, an Infection Form almost infected John-117.
 * In Halo Combat Evolved, if you hit an infection form with a Warthog, it would blow up and not cause any damage. In Halo 3, hitting the infection forms does not do anything to them, in fact, you can just drive over them, and it'll just make the ride a bit bumpy at the moment. However, the vehicle will sometimes lose traction and on rare occasions, the vehicle will flip over.
 * According to the book, Halo: The Flood, the Infection forms make a "meep" sound.
 * In Halo: Combat Evolved, if you shoot one infection form in a swarm, it will pop and cause a chain reaction causing all the other infection forms to pop with it in the swarm. This was taken out in Halo 2 but returned in Halo 3 (although not as powerful as in Halo: Combat Evolved.
 * Several can be seen within the Flood Observation chamber on the Halo 3 Multiplayer map Cold Storage. These Flood make Cold Storage the only multiplayer map with NPC enemies.
 * In Halo: Combat Evolved, the Infection Forms have a pulsating glow.