The Flood
The Flood, virulent, parasitic, whatever you want to call it. When Captain Keyes led marines into the Covenant ammo cache no one knew the horrors that they were about to unlock from the bowels of Halo. The Flood forms that leaped out at Master Chief were the first thing he really feared on that ring. Leaping, lumbering, twisting masses of decaying flesh and slippery tentacles are what becomes of anyone unlucky enough to be infested.
The infection form is as fragile as wet tissue, but they come hundreds at a time. A single form may appear harmless at first but if one comes in contact with a sentinent life being, sharp tenticles will penetrate the skin and drive into the victim's spine. From there on the host loses control of its body and the infection form 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. After it gains complete control over the host body the infection form will then inject Flood DNA into the host's system and begin the transformation process.
This is the Flood's combat form. Combat forms are created when an infection form takes over a physically strong body, such as a human marine or a Covenant elite. Their main form of combat consists of leaping (very high) or charging their target and then lashing out with tentacles (with strength that rivals even the Master Chief's) grown during transformation. Some combat forms even have the capability to wield the weapons dropped by victims and are often seen lumbering around holding a pistol or a rifle.
A carrier form is like the egg sac of the Flood. Small and weak hosts, like grunts or jackals, unsuitable to become combat forms are inflated during the transformation process. Eventually new infection forms grow inside and wait to be released. What happens most of the time is someone punctures the carrier form, which then explodes with the force of a fragmentation grenade, and then infection forms come spilling out. Master Chief (in the book) has noticed that the carrier forms still carry plasma grenades from their original state and uses that to his advantage.
Perhaps the Flood's most intelligently superior form, the brain form or command form, is the only form that can learn and absorb knowledge of planets that they could potentially infest. It's also what Captain Jacob Keyes of The Pillar of Autumn got turned into and in the game you can find "it" in the bridge of the Truth and Reconciliation, a Covenant battleship taken over by the Flood.
