Many disorders, drugs, and other treatments can cause a breakdown in the body's natural defenses. Such a breakdown can lead to infections, which can even be caused by microorganisms that normally live harmlessly on or in the body. A breakdown can result from the following:

Spotlight on Aging
Infections are more likely and usually more severe in older people than in younger people for several reasons: Aging reduces the immune system's effectiveness (see Biology of the Immune System: Effects of Aging on the Immune System). Many long-term (chronic) disorders that are common among older people—such as chronic obstructive pulmonary disease, cancer, and diabetes mellitus—also increase the risk of infection. Older people are more likely to be in a hospital or a nursing home, where the risk of acquiring a serious infection is greater. In hospitals, the widespread use of antibiotics allows antibiotic-resistant microorganisms to thrive, and infections with these microorganisms are often more difficult to treat than infections acquired at home or in the community.

• Extensive burns: Risk of infection is increased because damaged skin cannot prevent invasion by harmful microorganisms.
• Medical procedures: During a procedure, foreign material may be introduced into the body, increasing the risk of infection. Such material includes catheters inserted into the urinary tract or a blood vessel, tubes inserted into the windpipe, and sutures placed under the skin.
• Drugs that suppress the immune system: These drugs include cancer chemotherapy drugs, drugs used to prevent rejection after an organ transplant (such as azathioprine, methotrexate, and cyclosporine), and corticosteroids (such as prednisone).
• Radiation treatments: Such treatments may suppress the immune system, particularly when bone marrow is exposed to radiation.
• AIDS: The ability to fight certain infections decreases dramatically in people with AIDS, especially late in the disease (see Human Immunodeficiency Virus HIV Infection: Human Immunodeficiency Virus Infection). People with AIDS are at particular risk of opportunistic infections (infections by microorganisms that generally do not cause infection in people with a healthy immune system). Also, many common infections cause people with AIDS to become more severely ill.

Last full review/revision October 2012 by Allan R. Tunkel, MD, PhD