In 1997, The Lancet investigated three alleged cases of zombification in Haiti. The researchers diagnosed the victims with catatonic schizophrenia, epilepsy, brain damage, and potential fetal alcohol syndrome. A researcher from Harvard, Wade Davis, has speculated that some of the toxins administered by Vodou priests may make a seemingly dead person come back to life, albeit unable to function normally.
The Lancet investigated the case of a 30-year-old woman who died and was buried, then was seen walking in her village three years later. When she was found, the woman was mute and her family accused her husband of zombifying her. The researchers diagnosed her with catatonic schizophrenia.
The other two cases that The Lancet investigated were those of a man who appeared at a cockfight 19 months after he was placed in his tomb, and a woman who reappeared in her town 13 years after her death. The woman claimed to have been kept as a zombie slave 100 miles north of her village. The man was diagnosed with epilepsy and brain damage, and the woman with possible fetal alcohol syndrome.
Davis suggested that Vodou priests, known as bokor, can make a powder that includes tetrodotoxin, the toxin present in pufferfish. A sub-lethal dose of this toxin can make a person appear dead. While the heart slows and breathing becomes almost imperceptible, the person remains conscious. If the person is then buried, the bokor can exhume the body and administer Datura stramonium. Combined with oxygen deprivation, Datura makes the person disoriented and delirious, and possibly extremely passive.