Saint Domingue (Haiti), the western part of the once-Spanish island called Hispanola where Columbus had landed, was a colony of France. It produced coffee and sugar under the sweat and blood of imported African slaves. These slaves were brutally treated, and they kept themselves alive only with the aid of their religion. The Yoruba tribe in western Africa was largely responsible for carrying the belief in Vodu to the new world. (Voodoo was also known as Vodu or Vodun.)
In Saint Domingue, the Voodoo priests (or "houngans") and the paid-priests (or "bokors") had used Voodoo charms and potions as a form of biological warfare against the French who enslaved them, even poisoning their food supply on occassion. The Voodoo priests also drugged slaves who had betrayed the cause of slave revolution with Voodoo concoctions from natural herbs and from animal parts and held them as slaves. This is possibly the origin of the zombie.
The zombie was a resurrected body without a soul -- a social outcast who served the will of the Voodoo master. Supposedly, the zombie was raised from the dead, without free will or a soul. However, one modern theory is that the zombie never really died but was the victim of a drug. This Voodoo concoction is believed to have consisted of carefully selected herbs and animal parts, especially from the puffer fish, which contains a neurotoxin that causes a type of paralysis in the nervous system. The Voodoo priest also knew how to apply an antidote which could "resurrect" the zombie, but keep him dazed enough to be easily controlled. Most people, however, did not have the "magical" knowledge of the Voodoo priest. They believed the zombie was actually the living dead, a soulless body returned from the grave. Historically, Voodoo priests used to induce zombiism as a punishment for criminals; additionally, bokors could make someone into a zombie for a fee.
This belief of zombies weaved its way to New Orleans from Haiti as well, although zombies were not known in the Yoruba tribe in Africa. The belief in actual zombies was not as strong in New Orleans as in Haiti, but the term Zombi was certainly used in rituals, as evidenced by Marie Laveau's snake whose name (spoken in a Caribbean French patois) was Li Grand Zombi.