Infectious disease can involve bacteria, fungi, parasites, or viruses, all of which are microorganisms. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people. Of those that do cause disease in people, some are highly communicable (easily transmitted among 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.