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This is a diagram of the human body annotated with descriptions of the various barriers against infection. These comprise: intact skin and the acid in sweat; coughing and sneezing which expel infectious agents; enzymes in mucus, tears and saliva, in the nose and mouth; the mucus and intact mucous membranes of the respiratory tract; cilia in the respiratory tract, which trap foreign material; acid in the stomach; the mucus and intact mucous membranes of both the gut and the genital tract; competition from commensal bacteria in the gut and genital tract; and, in males, antibacterial proteins in semen.