Tetrodotoxin is a neurotoxin found in a wide variety animals, most notably the Japanese puffer fish fugu; however, there is good evidence that the toxin is actually produced by bacteria within these animals. For instance, fish grown in tanks do not produce toxin until fed tissue from toxin-producing fish 
The toxin binds to a subset of voltage-gated sodium channels in the central nervous system, preventing the channels from opening and so preventing nerve function. This activity has proved very useful in laboratory experiments to determine how nerves work.
Death by tetrodotoxin poisoning is rather rare throughout the world, but is most common in Japan where the flesh of the puffer fish is a delicacy. Between 1974 and 1983 there were 179 recorded deaths from tetrodotoxin poisoning in Japan 
It has been suggested that tetrodotoxin could account for the Haitian legend of the zombie, as the respiratory paralysis caused by the toxin could resemble death. After the toxin wears off, the victim would then appear to come back to life. However, other researchers have dismissed these claims.