An opportunistic infection is an infection by organisms that normally do not cause clinically significant conditions although the organisms may be normally present in the patient. An example of this would be an infection caused by a proliferation of bacteria that normally inhabit the intestinal tract. Opportunistic infections usually occur when the patient's immune system is depressed by cytotoxic drugs, radiation therapy, immunosuppresive drugs, or indwelling catheters in the artery (arterial line), or vein (central venous line, Swan-Ganz catheterization).