Botulinum neurotoxin, produced by Clostridium botulinum bacteria, is the most toxic substance known to humanity. An amount as small as 1 nanogram per kilogram can be lethal. Tetrodotoxin (from pufferfish) is extremely toxic but less potent than botulinum. Potassium cyanide is toxic but far less potent than either. Diphtheria is a bacterial disease, not a specific toxin.