Humans have evolved along with bacteria and viruses for millions of years, so it should be no surprise that the human body has developed a system of keeping the harmful microbes out. The first line of defense is the skin, which is a sheath of closely interlocking cells. Sweat and oil glands under the skin secrete acids that prevent the growth of microbes, and harmless bacteria that normally live on the body defend their territories against foreign bacteria.

The nose, mouth, and throat are common sites of attack, but fragile membranes and layers of sticky mucus that are toxic to harmful microbes protect them. Tears and saliva also contain antiseptic substances. Bacteria and viruses are sneezed out, coughed up, and cried out of the body. If microbes make it through these barriers, they are swallowed. Most will not survive in the stomach's...