Botulinum toxin
Botulinum toxin produced by "C. botulinum" is often believed to be a potential bioweapon as it is so potent that it takes about 75 nanograms to kill a person ( of 1 ng/kg, assuming an average person weighs ~75 kg); 1 kilogram of it would be enough to kill the entire human population. For comparative purposes, a quarter of a typical grain of sand's weight (350 ng) of botulinum toxin would constitute a lethal dose for humans.