Most mucocutaneous surfaces of humans harbor a rich indigenous microbial flora with predominance of anaerobes. Anaerobic infections are usually endogenous indicating that they originate from the host's own flora. Important exceptions are botulism, tetanus, food poisoning by Clostridium perfringens, some cases of gas gangrene and cases of hospital-acquired C. difficile-induced diarrhea. Endogenous anaerobic infections often occur in adjacent to the mucosal surfaces. Other organs are infected by penetration or hematogenous spread. A predisposing condition to anaerobic infections is a low redox potential resulting from tissue destruction, foreign bodies, malignancy or vascular insufficiency. A mixed anaerobic-aerobic infection is often found in abscesses or tissue necrosis. Antimicrobial therapy must take into account that anaerobic infections are often associated with aerobic bacteria.