Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Infections in People With Impaired Defenses


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jul 2020| Content last modified Jul 2020
Click here for the Professional Version
Topic Resources

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 (resident flora). A breakdown can result from the following:

  • 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), corticosteroids (such as prednisone), and biologic agents (such as adalimumab and others used for diseases such as ulcerative colitis, Crohn disease, rheumatoid arthritis, and other autoimmune diseases).

  • 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. 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.


Spotlight on Aging: Infections

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 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 (see Hospital-Acquired Infections).

Drugs Mentioned In This Article

Generic Name Select Brand Names
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Actinomycosis is a chronic infection caused mainly by the bacteria Actinomyces israelii. These are anaerobic bacteria, meaning they can thrive without oxygen. Actinomyces israelii are normally found on tooth enamel, gums, tonsils, and membranes lining the intestines and vagina. Under which of the following conditions does an actinomycosis infection typically develop?
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID