Clostridium consists of around 100 species that include common free-living bacteria as well as important pathogens. There are four main species responsible fordisease in humans:

C. botulinum, an organism producing a toxin in food/wound that causes botulism. Honey sometimes contains spores of Clostridium botulinum, which may cause infant botulism in humans one year old and younger. The bacteria produce botulinum toxin, which eventually paralyzes the infant's breathing muscles. Adults and older children can eat honey safely, because the Clostridia do not compete well with the other rapidly growing bacteria present in the GI (Gastrointestinal) tract.

C. difficile, can overgrow other bacteria in the gut during antibiotic therapy and cause pseudomembranous colitis.

C. perfringens, formerly called C. welchii, causes a wide range of symptoms, from food poisoning to gas gangrene. Also responsible for enterotoxemia (also known as "overeating disease" or "pulpy kidney disease") in sheep and goats.

C. perfringens also takes the place of yeast in the making of salt rising bread.

C. tetani, the causative organism of tetanus.

C. sordellii has been linked to the deaths of more than a dozen women after childbirth.

Clostridium is sometimes found in raw swiftlet birds' nests, a Chinese delicacy. Nests are washed in a sulfite solution to kill the bacteria before being imported to the U.S.

Neurotoxin production is the unifying feature of the species C. botulinum. Seven types of toxins have been identified and allocated a letter (A-G). Most strains produce one type of neurotoxin but strains producing multiple toxins have been described. C. botulinum producing B and F toxin types have been isolated from human botulism cases in New Mexico and California. The toxin type has been designated Bf as the type B toxin was found in excess to the type F. Similarly, strains producing Ab and Af toxins have been reported. Organisms genetically identified as other Clostridium species have caused human botulism; Clostridium butyricum producing type E toxin and Clostridium baratii producing type F toxin. The ability of C. botulinum to naturally transfer neurotoxin genes to other Clostridia is concerning, especially in the food industry where preservation systems are designed to destroy or inhibit only C. botulinum but not other Clostridium species.