Clostridium botulinum is an organism which produces toxins. It is ingestion of the preformed toxin (the most potent natural toxin known to man) that causes the illness botulism. No live organisms of Clostridium botulinum need to be consumed for foodborne botulism to occur. Spores of the organism present in food must be given the opportunity to germinate and grow to sufficient numbers to form the toxin. The toxin itself can be destroyed by heat treatment (80°C or above) for only a few seconds. Botulism is extremely serious and unless recognised and treated promptly, carries a high risk of mortality (35-40%). It is the most severe form of food poisoning.

Clostridium botulinum is ubiquitous - it is widely distributed in soil and marine sediments throughout the world. It is also found in the intestinal tract of animals, including fish. The organism can be isolated occasionally from many foods because of its widespread occurrence in the environment.

Most of the outbreaks of botulism have been associated with products of fish or marine animals (from which the highest incidence of isolation of Clostridium botulinum occurs), meat, fruit and vegetables - including mushrooms. Insufficiently heated canned and bottled foods are at high risk as these provide the anaerobic (without air) environment required by the organism to grow. Danger occurs if they have not had the necessary heat treatment to ensure the destruction of the spores. Other inadequately processed products such as farm-cured pork products or those produced where process control is insufficient e.g. traditional fermented products, have also been implicated in outbreaks.

Some strains of Clostridium botulinum are psychrotrophic, being capable of slow growth and toxin production at low temperatures (as low as 3.3ºC). Non-proteolytic strains of Clostridium botulinum are psychrotrophic. It is because of their non-proteolytic characteristics, their growth in foods cannot be detected by off-odours and off-flavours. The risk of toxin production prior to the food becoming unacceptable to the consumer is considerably higher than in those foods contaminated with proteolytic strains.

Clostridium botulinum is a spore former and it is in it's spore form that it is resistant to heat treatments such as pasteurisation. A heat process called a "Botulinum Cook" is required for low acid canned products.