Introduction
Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and produce illness in people.
Many microorganisms live on the skin and in the mouth, upper airways, intestine, and genitals (particularly the vagina) without causing disease. Whether a microorganism lives as a harmless companion to a person or invades and causes disease depends on the nature of the microorganism and on the state of the person's natural defenses.
 See the sidebar Types of Infectious Organisms.
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