Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency disorders usually result from use of a drug or from a long-lasting serious disorder (such as cancer) but occasionally are inherited. Immunodeficiency disorders impair the immune system’s ability to defend the body against foreign or abnormal cells that invade or attack it (such as bacteria, viruses, fungi, and cancer cells). As a result, unusual bacterial, viral, or fungal infections or lymphomas or other cancers may develop.
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