Botulinum toxin is the single most poisonous substance known.
Aum Shinrikyo tried but failed to use botulinum toxin as a biological weapon. Botulinum toxin is derived from the genus of anaerobic bacteria named Clostridia . It does not penetrate intact skin. The toxin irreversibly binds to peripheral cholinergic synapses, preventing the release of the neurotransmitter acetylcholine from the terminal end of motor neurons. This leads to muscle paralysis, and in severe cases, can lead to a need for mechanical respiration.