Immune Deficiency Disease, any one of a group of diseases caused by the impairment of the immune system. People with an immune deficiency disease typically become dangerously ill from the invasion of microorganisms that do not seriously harm healthy people. Immune deficiency diseases are characterized by frequent, often life-threatening infections, most commonly in the respiratory system, ear, or intestines. Other common problems are organ or bone infections, blood infections, and meningitis (infections of the brain membranes)

Previously healthy individuals who develop certain illnesses or undergo surgery or drug therapy may develop impaired immune systems. Trauma from severe burns or alcoholism that damages the liver can result in immune deficiency. Drugs used to suppress the immune system such as corticosteroids, those used before an organ transplantation, or radiation therapy, may also hurt the immune system.

Infections that cause immune deficiency include acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV). AIDS causes a progressive depletion of T lymphocytes so that there are not enough to fight infections. Low T-lymphocyte levels also make the body vulnerable to a host of infections that would not develop in a person with a healthy immune system. These opportunistic infections include Pneumocystis carinii, a type of pneumonia, toxoplasmosis, and fungal infections

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