As seen in ...
The movie The Serpent and the Rainbow, the upcoming Resident Evil 5 video game.
What are they?
There are certain kinds of poisons that slow your bodily functions to the point that you'll be considered dead, even to a doctor (okay, maybe not to a good doctor). The poison from fugu (Japanese blowfish) can do this.
The victims can then be brought back under the effects of a drug like datura stramonium (or other chemicals called alkaloids) that leave them in a trance-like state with no memory, but still able to perform simple tasks like eating, sleeping, moaning and shambling around with their arms outstretched.
How it can result in zombies:
"Can?" How about "does."
This stuff has happened in Haiti; that's where the word "zombie" comes from. There are books about it, the most famous ones by Dr. Wade Davis (Passage of Darkness and The Serpent and the Rainbow). Yes, the movie The Serpent and the Rainbow was based on this guy's actual science stuff. How much of it was fact? Well, there was that one scene where they strapped the guy naked to a chair and drove a huge spike through his balls. We're hoping that part wasn't true.
What is definitely true is the story of Clairvius Narcisse. He was a Haitian guy who was declared dead by two doctors and buried in 1962. They found him wandering around the village 18 years later. It turned out the local voodoo priests had been using naturally occurring chemicals to basically zombify people and putting them to work on the sugar plantations (no, really).
So, the next time you're pouring a little packet of sugar into your coffee, remember that it may have been handled by a zombie at some point.
Chances this could cause a zombie apocalypse:
On the one hand, it's already fucking happened! So that earns it some street cred right off the bat. But, even if some evil genius intentionally distributed alkaloid toxins to a population to turn them into a shambling, mindless horde, there is no way to make these zombies aggressive or cannabalistic.