Tetradotoxin (Ta-tro-do-tox-in) is an extremely potent neurotoxin which will render an individual to an extreme near-death state where no respiration or pulse can be observed even with sophisticated equipment. After several hours, the victim will "reanimate". It should be noted that an extremely small amount is needed to kill someone, thus, an even smaller amount must be administered to mimic the effects of death and subsequent belief that the victim has returned from the dead and is then a zombie.
The toxin is harvested from a number of marine fish including but not limited to, the pufferfish and triggerfish. Tetradotoxin has only been recently synthesized in the laboratory in 2003.
It should be noted that unlike Solanum and other fictional zombie viruses, tetradotoxin is quite real and deadly.