The gram-negative, anaerobic bacterium Clostridium botulinum produces the botulinum neurotoxin. There are eight strains of this neurotoxin: A, B, C1, C2, D, E, F, and G. All of these neurotoxins are paralyzing except for C2. Types A, B, E and rarely F and G, are associated with human botulism. The botulinum toxin was observed in spoiled sausages, which caused food poisoning. Hence, the term "botulinum" is derived from the Latin word for sausage, "botulus".

The bolulinum neurotoxin, when ingested with spoiled food, is spread through the blood (hematogenously). The toxin blocks the release of the neurotransmitter, acetylcholine, by traveling to the peripheral cholinergic nerve terminals.