Under normal circumstances, many bacteria live in coexistence with humans. The skin, digestive tract, upper respiratory tract, external urogenital organs, and conjunctiva all contain bacteria and many of them are commensal and do not cause disease. The presence of bacteria on or in these organs is not a threat to the body because the lumen of nasal and oral cavity, airway, digestive tract, and urogenital organs are connected to the “external environment” and are thus secluded from the normally sterile “internal environment.”