Zombies have existed for — well, for a very long time. The Epic of Gilgamesh mentions them, briefly, when an angry Ishtar says:

Father give me the Bull of Heaven,
So he can kill Gilgamesh in his dwelling.
If you do not give me the Bull of Heaven,
I will knock down the Gates of the Netherworld,
I will smash the doorposts, and leave the doors flat down,
and will let the dead go up to eat the living!
And the dead will outnumber the living!
It will be awful!

Generally any history of zombies includes mention of Haitian folklore in which bodies were brought back to life by bokor, or sorcerors. These zombies were mindless slaves, more or less harmless unless they were fed salt, which awakened their senses and made them rather dangerous. They would then either attack the bokor who had created them, or return to their grave and die.

In 1980, a man appeared in a village in Haiti who claimed to be Clairvius Narcisse, who had died in a hospital in 1962. He claimed to have watched doctors actually pull the sheet up over his face and declare him dead, and he believed that a bokor was controlling him. As his death was documented by the hospital, scientists came to believe that this might be proof for the existence of zombies in Haiti. This was enough to spark the Zombie Project, lead by anthropologist Dr. Wade Davis in Haiti from 1982-1984. Davis found that the ingredients in the powders used by the bokor to create zombies included tetrodotoxin, which caused the people who ingested it to go through a temporary paralysis. This made them unable to respond to stimuli for a period of time, but eventually they experienced full recovery. A few of the other consistent ingredients were bits of marine toad, hyla frog, and human remains.

Another tidbit of note: it makes little to no sense for zombies to eat human brains, as they're sort of small and not a fantastic source of food — it is believed, then, that the brain-eating bit is only the result of the zombie in question being controlled by someone else. Of course, George A. Romero's films have suggested that some chemical property in the human brain serves to numb the pain of existing as "undead," which is why zombies are prone to nosh on the grey matter from time to time.

This is, of course, simply to provide you, dear zombie hunter, with the proper background to face the theat that zombies present, provided you've had time to read thus far. If you are currently in a state in which immediate termination and disposal of one or several zombies is absolutely urgent, please skip back to the Speedy Action section.