Anaerobic bacteria are bacteria that do not live or grow when oxygen is present. In humans, these bacteria are most commonly found in the gastrointestinal tract. They play a role in conditions such as appendicitis, diverticulitis, and perforation of the bowel.

Obligate anaerobes are major components of the normal microflora on mucous membranes, especially of the mouth, lower gastrointestinal (GI) tract, and vagina; these anaerobes cause disease when normal mucosal barriers break down.