Anyways, before answering the actual question I have to drop a bit of knowledge so that the educated people who read this won't think that zombies are alive and real in Haiti.
Zombification was a process that came about in the days of slavery on the island. Slaves who brought to Haiti from Western Africa were not allowed to practice their own native religions, speak their own language or even stay together with their families. In response to this, the Africans along with the native Taino of the island came up with a way to rebel. It was voodooism. It constitutes people being able to talk to their dead relatives as a messenger to get through to God. Sort of like the Christians idea of what an angel is. It also involved using eastern medicine to protect themselves from diseases carried over from the British/French and Spanish coming to their islands. Eventually as slavery got worst and worst Voodoo was twisted into something evil and a Black Magic of sorts came out of it. (there's always good and bad of course).
Anways... The legend says that the "Bokor" or the "Voodoo Sorcerers" had the ability to make someone a "zombie" and have power over that person. what's used to make "zombies" is this powder called coupe poudre.
The active ingredient in coupe poudre is tetradotoxin which is found in the liver and the ovaries of some species of Puffer Fish. It's more deadly than even Cyanide. Tetradotoxin is a potent ion channel blocker which while it can be fatal, in smaller doses leads to a near-death state wherein metabolic functions are so depressed that the poisoned person is thought to be dead. Tetrodotoxin specifically and reversibly binds to a pocket on the outside of the sodium-ion channel in the peripheral nerves. This blocks the channel so that Na+ cannot enter, thus preventing the reversal of polarity that constitutes the action potential effectively shutting down the propagation of the nervous impulse. So the results of all this is that total body paralysis is induced, although the brain and senses stay alert.
Along with that, several other types of poisons have reportedly been detected in coupe poudre. Datura metel and Datura stramonium, both known as "zombie cucumber," are two such species of plants, which are hallucinogenic and cause amnesia. Another species used in the powder is Mucuna pruriens, a plant with stinging hairs and one which contains "psychomimetic constituents and may have hallucinogenic activity. These poisons constitute the powder, which is surreptitiously administered to the skin of the intended victim. But, the horror does not end there. The effects of the coupe poudre wear off in about 10-12 hours. The victim is then disinterred and fed a paste made of atropine and scopolamine, dissociative hallucinogens that impact on the neurotransmitters and endorphins in the brain.
In Haitian voodoo imaginary a zombie is revived without retaining his soul, or his self-consciousness, or indeed, his mind. But, leaving aside the religious-laden issue of whether or not one has a soul, death is the end of life and consciousness. The voodoo zombie is not in fact then a revived dead person, but a living person who has been brain-damaged
They used this on the people coming to the island enslaving them as well as on each other because for one, it scared people away for fear of zombies and for two if the slaves and the natives of the island are useless, their hope was that the people who go away and leave them alone.
As far as I know it still goes on in most of the islands in the Caribbean. For Haiti specifically the Haitian Penal Code lists making someone into a zombie a form of murder.