I've always liked this theory:

The bacteria has been around for a long time, probably originating in west Africa and for a period existed in the Caribbean and was associated with Voodoo. At the beginning of the 20th century it was almost wiped out by the introduction of antibiotics yet it never completely went away. Over the next 100 years the bacteria had an unknown natural reservoir, a host mammal of some sort unaffected by it, and gradually developed a slow resistance to antibiotics (possibly those used in industrial farming). An unhappy coincidence was the bacteria also developed a long incubation time during this period with symptoms developing in humans only after 6-12 months from exposure. This allowed the bacteria to spread globally, probably due to international trade and/or travel, and enabled it to affect people on all continents before the first recorded outbreak. Transmission is possible through the respiratory system and direct contact and, due to the incredible survivability of the bacteria on un-disinfected surfaces, a large number of people were infected in those first 6 months. No symptoms are present in healthy individuals. The pathology of the bacteria is particularly interesting as it primarily rests in the nervous system and brain-stem of its host and more importantly flourishes in the low redox environment of tartar that lies between human teeth. The immune system seems to be able to control the bacteria successfully in healthy individuals however at the moment of death of the host the bacteria is able to multiply exponentially into enormous quantities. Re-animation occurs at varying times depending upon initial levels of bacteria in the infected individual. These heightened levels of bacteria mean a bite from a reanimated individual transmits a dose of bacteria sufficient to overcome the immune system of even the healthiest individual resulting in imminent death and re-animation. Instant removal of bitten limbs has been shown to halt the transmission of the bacteria to the brain-stem with a relatively high survival rate. It has been shown that some form of chemical signal can be detected by the bacteria when present in sufficient numbers. This enables the bacteria to identify successful mass-infections and goes some way to explain its ability to differentiate between healthy humans and the mass-infected. This is assumed to be part of the natural life cycle of the bacteria as it results in a net increase in total bacteria numbers. Infected individuals will continue to function (in terms of basic movement and desire to feed) unless catastrophic damage is done to the brain. Total removal of the head initially proved effective yet it was discovered later that a severed head can remain animated if the brain remains intact and the brain-stem undamaged. Studies suggest that infection of corpses is impossible as there are no recorded events of corpses re-animating that died 12 months after the initial outbreak.