If you travel through an airport during a major holiday season, you are very likely exposed to a ...

 The human body depends on several chemical and physical barriers to block pathogens from entering the body. Among these defenses, the skin is of paramount importance as it is a physical barrier, which is reinforced by the protein keratin being deposited in the outer layer. Skin also releases acidic secretions that can kill many microorganisms, and the resident bacteria living on skin can also aid to keep out nonresident microorganisms. Other body fluids like tears, vomit, digestive fluids, mucus, and urine all function as chemical and/or physical barriers.

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