Opportunistic infections are so named because they occur in people whose immune systems are not working properly; they are "opportunistic" insofar as the infectious agents take advantage of their hosts' compromised immune systems and invade to cause disease.

The organisms that cause opportunistic infections are categorized as protozoa, fungi, viruses and bacteria. These organisms are found widely in nature and often live in the human body. When the immune system is working properly, it can control the germs. However, persons with defective immune systems are unable to fight off the growth and destructive action of these organisms within the body. Opportunistic infections are seldom spread to people who have normal healthy immune systems.

One of the ways the immune system can be damaged is when the person is infected by HIV. In addition, drugs used to treat cancer, and drugs used to facilitate organ transplants can...