Munchkin Zombies [http://www.worldofmunchkin.com/munchkinzombies/] (fun game) categorizes them as Atomic, Voodoo and Plague.
Voodoo Zombies are the most classical, as the term zombie actually comes from the Creole term of a person raised from the dead by a magical technique (some variants, the technique kills a live person and brings them back zombified). Usually undead zombies are the slow lumbering type, and are not felled by damage that would otherwise kill (e.g. end vital signs) so that zombie limbs or partial bodies will continue to lumber forth. Magical zombie apocalypses come when spells go wrong, when an overlord decides to raise an undead army, or simply due to it being an end-times prophecy to some sects. (I think Revelations says something to this effect.)
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Atomic Zombies come from the early era of the cold war, when the effects of nukes and radiation were not entirely known to the public (much like genetically engineered foods or nanotech today) and hence they were given magical effects by Hollywood. Radiation could give living people superpowers and raise the dead back to life. We see a throwback to this in the game Singularity, where standard alpha-particle radiation is replaced with the unknown energies of element E-99.
Then there's the more recent Plague Zombies, which is certainly the most likely case for a zombie outbreak. Left4Dead treats it as a rabies supergerm, where Zombieland stated it was a mutation of mad cow (indeed, prions are infamously difficult to track or contain). The 28DL virus was an engineered biowarfare drug. In all such cases, zombies the infected are still very much alive, but are usually immune to pain, and don't go into shock easily. Usually, all the other zombie tropes are justified in the zombie behavior (e.g. in L4D, a fallen cadaver often reveals to be a very still resting infected, hence they rise from the dead).
Not yet mentioned, are parasite zombies, much like the Half-Life headcrabs which will attack and infest a host, taking control of it at the brainstem. What is particularly scary is that there really are such creatures [http://en.wikipedia.org/wiki/Dicrocoelium_dendriticum], albeit in the ones that take control of their hosts usually do so to feed it to another creature (who then becomes the new host). We've seen many body-snatcher stories in which people become parasite zombies, yet only in Half-Life have I seen those infected associated with zombieism.
The last form is active mind-control zombies as seen in the most recent version of The Mummy. Like the followers of Imhotep, mind-control zombies seem to be cultists or other followers of a dogma or ideology. Hmmm...sounds like symbolism.
238U
