Many disorders, drugs, and other treatments can cause a breakdown in the body’s natural defenses Defenses Against Infection Natural barriers and the immune system defend the body against organisms that can cause infection. (See also Lines of Defense.) Natural barriers include the skin, mucous membranes, tears, earwax... read more . 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 Resident Flora Healthy people live in harmony with most of the microorganisms that establish themselves on or in (colonize) nonsterile parts of the body, such as the skin, nose, mouth, throat, large intestine... read more ). 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 Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS). HIV is transmitted... read more : 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.
(See also Overview of Infectious Disease Overview of Infectious Disease 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... read more .)