https://science.howstuffworks.com/science-vs-myth/strange-creatures/zombie1.htm
Tetrodotoxin causes paralysis and death, and victims of tetrodotoxin poisoning often remain conscious until just before death. The paralysis prevents them from reacting to stimuli – much like what Clairvius Narcisse described about his own death. Doctors have also documented cases in which people ingested tetrodotoxin and appeared dead but eventually made a complete recovery.