The Walking Dead. Scientific name Homo Coprophagus Somnambulus.
A deceased human being who has partially returned to life due to undeterminable causes. The brain retains base facilities, namely gross motor function. In its near-mindless state, it grasps no remains of emotion, personality, or sensation of pain. In rare cases, some of the reanimated have reflexively preformed routine activities from their past lives.
The rotting bodies of the undead operate on a fraction of the level at which our bodies normally function. Circulatory, respiratory, and digestive systems are unaffected by reanimation. Labored breathing, choking, and moaning are reflexive but no oxygen is carried through the blood. The nervous system functions primarily within the brain and brain stem. Sensory reception is minimal at best and seemingly unecessary in the pursuit of prey. The undead are incapable of fatigue and will persist at any cost. They will even crawl when their legs have been removed. Even if the head is removed from the body, it will continue to live. The only way to stop the reanimated is to destroy the brain. To prevent reanimation in the recently departed, decapitate the corpse and burn the body.
The only observable action a zombie takes part in is killing living creatures, especially humans, and eating them. Many theories and speculations surround this disturbing behavior. One theory is based on the thought that reanimation is the result of a contagious infection or virus, and that the primal drive to feed will spread the disease to other host bodies. Research has shown that although the majority of zombie attacks result in fatal wounds, all corpses return to life soon after passing, regardless of cause of death. Another theory is that zombies eat the brains of the living to refuel the "un-life" giving chemical serotonin. Because digestive and circulatory systems are incapable of bringing these elements to the brain, this just cannot be true. The final speculation seems the most obvious, that the dead feed for sustenance to satiate their unnatural metabolism. But because the gut has no function in the undead, this is also false. One documented encounter claims that a zombie was unable to move due to the sheer mass of undigested flesh resting in its distended gut. The creature continued to eat even after it's gut had burst open. Studies regarding the nature of feeding have proven that zombies will try to eat when their stomachs and even jaws have been removed. One explanation offers that the walking dead are the incarnation of death itself, a mockery of life that uses the vessels of the living to carry out their dark intentions, they are the opposite of life and are driven to simply undo it.
"When there's no more room in hell, the dead shall walk the earth."