h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I would just like it to be noted that exerpts from the Necronomicon do NOT create zombies in the truest sense. Reading from this unholy tome merely allows spirits of the dead to be able to posses the living. The result is pretty much the same as standard zombification, although these possesed humans don't rely on eating human flesh to survive.

The most common reason for zombie transformation is a chemical developed by the Dara chemical company called 245-Trioxin. The original purpose of this chemical is unknown but a noted side effect is that whether in gaseous or liquid form, it is able to re-animate the dead causing them to move around and have a tremendous desire the eat the brains of the living. These chemically created zombies are notoriously hard to kill, destroying the brain is not enough, you have to destroy these guys entirely to stop them. It is also to be noted that destroying them by incineration is NOT recommended as the chemical drenched fumes created when these zombies are burned is sufficient to re-animate any other corpses within the immediate area.

Voodoo incantation is another method of creating zombies. This would usually be done by a voodoo priest or priestess but unlike the zombies created by application of 245-Trioxin, these undead creates retain most of their faculties, have no desire to eat the brains (or flesh) of the living and are merely slaves who serve the bidding of the voodoo priest. Unlike the 245-trioxin created zombies, these undead creatures can be killed relatively easily by decapitation or by destroying the brain.