Despite the vast differences in many undead species, there appears to be only two ways in which the dead can become undead.
Most often referred to as the
resident evil school of thought, this is the belief that the undead are created when a formerly unknown pathogen infects a living or deceased host, kills them (if they are still alive) and then uses their body as a breeding ground and mobile incubator to sustain a large number of the same pathogen. In the case of zombies, the victims deceased brain, which is now only capable of lower functions, drives the lumbering mass of death to seek out new host organisms to infect, thus perpetuating a deadly cycle of infection and re-infection. In the case of vampires, where a blood fluid exchange takes place between the hunter and its prey, an unknown pathogen is passed seemingly unnoticed between the two that gradually evolves the victim into a similar creature of the night. However, while death does occur in vampires, the brain, unlike in zombies, remains entirely unharmed. This allows the vampires to retain their memories and identities which are lost when the outer surface of the brain begins to decompose.
Magic and Necromancy/Supernatural Means
While infection represents a specific scientific take on the undead, Magic and Necromancy represents a much broader spectrum of explanations, and because such things are not at present proven to exist, the means in which they reanimate the living is impossible to determine. This vagueness does have its bonuses, however (depending on who you are). Any method that cannot be explainable by science is categorized as having occurred through supernatural means. Phantoms, wraiths, spirits and ghosts are all unexplainable by science. However, these creatures continue to frequently pop-up in games. The only way a person can rationalize their existence is to believe that they were created by supernatural means, outside the laws of science.