According to the Romero rules of Zombieism
a zombie is created in the following ways:
-The people that are already dead at the time of the first inception of zombie creation will return as zombies- presumably because they are exposed to whatever creates zombies. This may be caused by something created by scientists that escaped or was intentionally released, such as a virus, or something like nuclear waste contaminating the soil. It is not clear whether only a group of corpses were effected, or whether every single dead body world wide becomes a zombie at this point.
- A living person that gets bitten by a zombie, upon death will return as a zombie. It is not clear whether the bite itself causes death. Presumably any other unrelated cause of death of a bitten person will still result in them returning as a zombie; also, since a disabling of the brain will kill a zombie, it's not clear whether disabling the brain of a bitten person while they are living will effect whether or not they come back as a zombie, but logically it would make sense that they then would not return as a zombie.
- When the original wave of zombies are burnt whatever caused the creation of zombies will be transferred back into the soil by water. It is unclear whether this is caused by the water mixing with the ashes of the zombies, or whether the burning process causes contamination from the rain itself. At this point any corpses that were not effected by the first zombie inception and/or people that died since that time that were not bitten by zombies, will then return as zombies. Presumably exposure to the contaminated water alone will not cause living people to turn into zombies.
- A scientist using whatever created the zombies initially, or recreating it through experimentation on the zombies can then create a zombie from a living or dead body.
* This is an assumption based on the events in the films and isn't actually depicted.
Note: It is not suggested by the films that a person who dies and hasn't been bitten by a zombie will return as a zombie, simply from having died. Only corpses that became zombies simultaniously during the first zombie creations, and living people subsequently bitten afterwards will become zombies.
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