From Halo - Wiki on Neoseeker

The Flood are an ancient race of creatures that were found to be living on the Halos in the Halo trilogy. The flood are first encountered when a group of marines enters into a room on Installation 04. The Flood are a parasite that rely on another host to survive and once they do their outward appearance is similar to that of a zombie. They can resurrect fallen comrades and enemies alike and force their bodies into motion by invading the person's nervous system. There are multiple kinds of Flood that are encountered in all three of the games as well as the novels.

Contents 1 Forms 1.1 Infection 1.2 Combat 1.3 Carrier 1.4 Pure 1.4.1 Ranger 1.4.2 Tank 1.4.3 Stalker

Forms

Since Halo different forms of the Flood have showed up during the course of the game.

Infection

These little critters look harmless enough alone since they pop from a simple Assault Rifle burst, but don't let their looks fool you. These guys are known for their names. They are the main causes of all the other forms of the Flood. They infect a sentient being by injecting part of themselves inside the host and taking them over. It doesn't matter if the targest in question is dead or alive, the Infection form will force them to do its will. Also in a group these buggers can swarm over Master Chief and quickly deplete his shielding.

Combat

These forms are, as their name implies, the main adversary that the Flood brings about. They come in alternate forms depending on who the Infection form took over. Infected Humans are usually thinner flood with smaller limbs and are easier blown apart. Infected Elites usually still carry their old armor, and are generally the most annoying Flood Combat form. The shields still sometimes function, making them harder to kill when in a big pinch. And the Brute Combat forms are usually bigger and more terrorizing than most of their counterparts. But they still all fall just the same.

Carrier

These are one of the more annoying Flood forms. They are just as their name implies, carriers. They walk on stubby legs while the top half of their body is made of a bulbous and stretched skin-like material. Unfortunately, it acts the same as an infection form when a round is loaded into it, it explodes; but this is a bad thing, it releases a ton of Infections forms that just swarm and end up hurting you just the same as the explosion. The best thing to do to Carrier forms is to stick them with Plasma Grenades or Spike Grenades.

Pure

These pure forms are a new addition to Halo 3 and are tougher than your run of the mill Flood. They are not actually one form, instead, they are one Flood that is able to turn into 3 separate types of enemies; and each of them packs a punch in their own way.

Ranger

The Ranger form is exactly how it sounds, it's a ranged attacking Flood form. It attacks from a long distance, mounting itself on walls, grounds, ceilings, or just about everything. They're vicious bastards that shoot out sharp needles that tear into energy shields as well. They are also relatively tough to kill with most weaponry, but the most effective happens to be the Needler.

Tank

The Tank form is a pretty beefy and powerful form of Flood. It looks almost as if a Hunter was infected by the Infection forms. It's got a thick limbs and attacks with a charge similar to the Brute, which can end up sending you flying. The usual best result to take out these are assassinations or sticks. Your usual best bet is to run for cover until they happen to change shape again, since most of your hits do nothing to combat its thick hide.

Stalker

Stalker forms are their quick and agile pure forms. They look similar to spiders and can sure run away. They have a great leaping ability which is used to attack, or they get far enough away to get themselves in a Ranger form and take you down. These are the weakest of the Pure forms, since they have really no defense and rely basically on their speed. Get 'em down fast and you're safe.

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