Zombies have existed for some time. However, I judge by your post that you are referring to the "undead", and not the Voodoo created mentally challenged individuals.
Much like Dragons, nearly every culture has stories of the dead returning to life. It takes many forms, from Vampires, to Wraiths, to rotting corpses. While the idea is rather fanciful, It could very well be based in fact. There have been many things over the years originally thought to be simply myth, that in the end turned out to be true. I generally use the Tasmanian Tiger as my reference for this. but there are others. Angler fish, Japanese Wolves, Buru (still not technically proven, but many biologists concede the fact that the species most likely existed and went extinct). So a virus spread through body fluids that causes the infected maybe not to "die" per say, but to lose a lot of motor function and become terribly aggressive due to affecting the brain that arises in some backwater tribal area where oral tradition has handed down the tale of these things isn't so terribly unbelieveable. It may even trigger a release of a massive amount of Adrenaline due to it's effect on the brain. A person with enough of that chemical flowing through their body could feel no pain, and usually debilitating injuries seem to have absolutly no effect on them. I've seen it happen. (In Iraq, not with zombies). Our complete reliance on conventional (and fallable) science leaves us unable to accept this though. "If that was true, we would have heard of it already." But is that so true? How long did it take for people to hear of H5N1 a.k.a. Avian Flu? Or the new species of Man-o-war discovered near Japan in 2005? How about the red rain in India in 2006?
An open hand travels faster than a closed fist. The same is true for the mind.
