A Zombie is a re-animated corpse, usually brought back to life by an alien parasite or bioweapon. "Zombie" (also spelled "Zombi") used to refer to a corpse possessed by a voodoo snake-demon under a witch-doctor's control and kept in rather good condition; however horror films have changed the meaning to mean any creature that rises from the dead for whatever reason, shambles about, rotting on its feet, and eats living members of its species. The Zombie legend are often confused with several other undead legends, including Ghouls ([Arabia] Bite turns the victim into another one, cannibalistic), Anthropophagei ([Europe again] travel in hordes, also cannibalistic) and Revenants ([Europe], return from the dead to punish the living). Usually, the only way to kill them is to destroy the brain, as decapitation often results in a semi-living head.
Clinical death means that all cardiovascular and respiratory functions-breathing and blood flow, irrespectively-have ceased. Once this happens, the immune system shuts down, eliminating resistance to parasitic organisms and leading to decay. Brain damage occurs due to the buildup of toxins, and skin and muscles putrify due to the bloodstream neither feeding the tissue nor carrying away its waste materials. This dead, rotting state is a prerequisite to becoming a zombie, rather than simply being a person taken over by assimilating races.