The body's natural flora, membranes, and inflammatory response form a natural defense against infectious agents regardless of previous exposure.

The normal flora of the human body includes the microbes that inhabit the skin and mucous membranes.

They stimulate the production of natural antibodies, destroy microbial invaders, prevent pathogen colonization, and help to synthesize vitamins.

The skin is also a distinctive defense against infection. It is a physical, natural barrier that prevents the invasion of microorganisms.

Similarly, the eyes produce tears that contain lysozymes, which protect them from infection.

Additionally, the cilia lining the airways are coated with mucus that captures inhaled microorganisms enabling the cilia to move them outward to be swallowed, coughed, or blown out of the nose.

Stomach acid, pancreatic enzymes, bile, and intestinal secretions kill bacteria and other invaders.

By moving materials constantly, small intestinal peristalsis inhibits microbe retention.

Similarly, when the bladder empties, the urinary system discharges bacteria.

Finally, vaginal acidity inhibits harmful bacteria and preserves normal flora.