Almost all anaerobic infections originate from the patient's own microflora. Poor blood supply and tissue necrosis lower the oxidation-reduction potential and favor the growth of anaerobes. Any condition that lowers the blood supply to an affected area can predispose to anaerobic infection. Therefore, foreign body, malignancy, surgery, edema, shock, traum and vascular disease may predispose to anaerobic infection. Previous infection with aerobic or facultative organisms also may make the local tissue conditions more favorable for the growth of anaerobic organisms. The human defense mechanisms also may be impaired by anaerobic conditions .