The flood are parasites that are all consuming. They will stop at nothing to consume you into their army of dead. The forerunner created the Halo rings because of these creatures. The Forerunners created the rings to be fired when the flood became to much. The rings can only be fired by the Humans. You never know where they may be hiding.

Human Flood- The weakest flood in Halo 3. These are the flood forms of infested humans.  All you pretty much have to do against them is beat them down. They  usually carry SMG's for weapons.The best way to kill them is to  beat them down or use duel plasma rifles.

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Elite Flood- The Elite flood put up a better fight than the humans. They wear armour and depending on what type may take many beat downs to kill them. They are tough. Use duel plasma rifles to kill them.

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Carrier Flood- When these flood fall down they explode, sending out around 10 infection forms. Don't get too close to them when they explode. The best way to kill the flood form is with double plasma rifles.

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The Gravemind is essentially the center-point of the flood. He is big and has betray the Master Chief many times.  He is currently on the ship High Charity.

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Brute Flood- The brute flood in Halo 3 is one of the more powerful flood. The brutes are large but get overpowered by the huge waves of flood. Soon the flood take over their host and feed on thier bodies until it is nothign but flood. The best way to kill these flood is with grenades or other powerful weapons.

 An infected Brute Chieftan

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Tank Form -  This is the most powerful form of flood. It will move its arm in front of its face to block your attempted headshots. It will also shoot infection forms out of its mouth. The best way to kill it is with an energy sword or flamethrower. These flood forms are new in the Halo games. Two plasma rifles are the best way to take these flood out.

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Ranged Form - These flood are stationary and shoot Brute Spiker like shots into you. They are highly annoying and dangerous. The best way to kill this type of flood is with grenades or a brute shot.

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 Infection Form- These forms can be seen crawling on many flood levels. Kill them quickly, for they can bring dead flood back to life. They latch onto the dead with their sensory appendages and slowly bring them back up from the hosts grave. Just shoot anything at these flood forms.

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 Stalker Flood- These flood can climb almost anything. They can even climb upside down. They do not put of a good fight,