Clostridial bacteria are anaerobic and usually cause diseases by producing a toxin. Tetanus, botulism, and gas gangrene are examples diseases caused by clostridium.
Clostridium perfringens is found in soil and in the intestines of humans and animals. In contaminated food, it produces a toxin that causes a form of food poisoning.
Pseudomembranous colitis is a complication of antibiotic therapy caused when a normal intestinal bacteria, Clostridium difficile, overgrows. The antibiotic kills other normal intestinal bacteria that keep Clostridium difficile in check.