Natural flora, body system defenses, and inflammation are natural barriers of the body against infectious agents regardless of previous exposure. Normal floras of the human body refer to the microbial population that colonizes the skin and mucous membranes.

In addition, many body organ systems have unique defenses against infection. The skin is an intact, multilayered surface preventing invasion by microorganisms unless impaired. Mucous membranes lining the mouth, nose, and eyelids are barriers against microorganisms. The cilia lining of the upper airway is coated with mucus, which filters inhaled microorganisms. The gastrointestinal tract includes stomach acid, pancreatic enzymes, bile, and intestinal secretions, which can kill and prevent bacteria from multiplying. Rapid peristalsis in the small intestine prevents the retention of bacterial contents. When the bladder empties, the urinary tract removes microorganisms from the bladder lining and urethra. Vaginal acidity prevents harmful bacteria from growing and helps maintain the optimal number of normal flora.