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• Biology of Infectious Disease
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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:

• 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 procedures include insertion of a catheter into the urinary tract or a blood vessel and insertion of a tube into the windpipe.
• Drugs that suppress the immune system: These drugs include cancer chemotherapy drugs, drugs used to prevent rejection after an organ transplant (such as azathioprineSome Trade Names
  IMURAN
  , methotrexateSome Trade Names
  TREXALL
  , and cyclosporineSome Trade Names
  NEORAL SANDIMMUNE
  ), 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). People with AIDS also become more severely ill from many common infections.

Infection From Medical Devices Usually, people think of infection as occurring when microorganisms invade the body and adhere to specific cells. But microorganisms can also adhere to medical devices (such as catheters, artificial joints, and artificial heart valves) that are placed in the body. Microorganisms may be present on the device when it is inserted if the device was accidentally contaminated. Or infecting organisms from another site may spread through the bloodstream and lodge on an already implanted device. Because implanted material has no natural defenses, the microorganisms can easily grow and spread, causing disease.

Last full review/revision October 2008 by Allan R. Tunkel, MD, PhD