Pod infector

The Flood Infection Form is 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 its 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. An Infection Form may also abandon its host if he/she has been critically injured, and look for a new one. Several Infection Forms, sometimes ranging from 5 to 15 Infection Forms at once, can attack certain victims. If a flood gets into an elite, it will still have its energy shield when you try to kill it. Also, if you shoot the middle of a flood marine or brute, it will die but the infection form can reanimate it and it will come back to life again.

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, it is also possible that he contracted the illness from the drugs he supposedly took on the SPARTAN-I Program. According to a UNSC analysis of various Flood forms, the Jackals and Grunts lack necessary calcium stores and biomass and are instead used as biomass stores. It should also be noted that in Halo 3, infection forms will ignore Jackals and Grunts most of the time and go directly to Brutes. 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, chitinous exoskeleton would seem to make them immune as well.

Halo: Combat Evolved

 * 343 Guilty Spark
 * The Library
 * Two Betrayals
 * Keyes
 * The Maw

Halo 2

 * The Oracle
 * Sacred Icon
 * Quarantine Zone
 * Gravemind (Final cutscene)
 * High Charity

Halo 3

 * Floodgate
 * The Covenant
 * Cortana
 * Halo

Deterioration
An Infection Form is capable of losing its infectious potential over time. This was experienced by Private First Class Wallace A. Jenkins during the raid on the "Covenant Weapons Cache" on Installation 04. This aged version of the Infection Form must have been kept in captivity for too long, thereby reducing its potential of complete transfer of its host into a Flood Form. Jenkins was only partially controlled, as there were periods of time where he could control his body to some degree.

Combat
Infection Forms will gather in large swarms, attacking with their claws and tentacles. If the victim's shields are active, as explained earlier, the Infection Form will burst. However, the sheer force of numbers can sometimes deplete a victim's shields, allowing these Flood to take control of the Human/Brute/Elite.

A single round from any weapon will burst an Infection Form. For this reason, automatic weapons are reccomended. Dual-wielding Plasma Rifles, Brute Plasma Rifles, Spikers, or using a UNSC ballistic weapon all work well. The battle rifle, with its bursts of fire, are especially effective. Try to avoid using weapons such as M6 handguns, or Plasma Pistols, as their rate of fire is very low. Also, try to avoid using explosives, as they close in extremely quickly-you may do more damage to yourself than to them.

Infection Forms are spawned from bulging, sac-like pods in Flood Hives. It may be tempting to destroy these sacs, but it is actually more advisable not to, since destroying them will not kill the Infection Forms inside.

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 in Halo 2 and 3, but 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 out Infection Forms. This is demonstrated during Halo 3 for the Tank Form and Halo 2 for Gravemind.
 * 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 of the Alien franchise and the Borg Nanites of the Star Trek franchise.
 * The Infection Forms in Halo 3 float in water. Thus, they cannot infect corpses that are in water ponds because of this animation. (E.g. on the level Halo it'll look different close up then it will farther away.) However, in Halo Combat Evolved they sink.
 * In the book Halo: The Flood, an Infection Form almost infected John-117 when he turned his back, due to his energy shields being drained.
 * 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 less effective 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 5th multiplayer map with NPC enemies (Along with Construct, Guardian, Last Resort, and Isolation which have Sentinels, and Constructors flying around). Unfortunately, you cannot get to them.
 * In Halo: Combat Evolved, the Infection Forms have a pulsating glow.
 * On 343 Guilty Spark, when you watch the video, the Infection Forms have holes in them.
 * In Two Betrayals, the Infection Forms feeding on the corpses of a Minor Elite and a Minor Grunt are stationary no matter what.
 * There is a glitch in Halo: Combat Evolved where Infection Forms can literally be stuck, such as in the level Keyes, the Infection Forms can sometimes get stuck in the middle of a shield, or behind a damaged wall on the ship.
 * Sergeant Johnson is immune to infection due to the fact that he has Boren's Syndrome.
 * Infection Forms spawned from pods can only live for a limited amount of time before they die, this is most likely due to the form not being developed enough to survive for long periods outside its pod.
 * Strangely, in Halo: Combat Evolved on levels such as The Library, Infection Forms will get stuck to walls and cannot come out, also sometimes as happened one Infection Forms get frozen on ground and in mid air.
 * The Flood Infection Forms are the weakest enemies you face in the Halo series.
 * In Halo 3, Infection Form swarms are capable of overturning a Warthog, making the Warthog run on Halo dangerous when they are encountered.
 * It appears that after a while, Infection Forms can age, as seen by the effects on Wallace A. Jenkins by an Elder Infection Form.
 * In Halo 2, the Infection form will sometimes get stuck inside a wall.
 * In Halo 2 if an Infection form attempted to infect a brute, the brute would just die and the Infection form would crawl off.
 * In Halo 3 if you are killed by an Infection form they will strangely leave your body.
 * If you kill Johnson before you kill sparks he will still be alive after the battle.
 * If Johnson is left to take care of himself, he will be turned into one of them!