If not broken, the skin is an impenetrable barrier to viruses and bacteria. It is reinforced by chemical defenses. Oil and sweat give the skin a pH of 3-5. Bacteria that live on the skin help by releasing acids and wastes to kill off unwanted microorganisms. Lysozyme is found in tears and saliva which can destroy the cell walls of most bacteria. Gastric juices kill off bacteria in food. Mucus captures bacteria in the nose and throat cavities of the body.
