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Immune deficiency diseases may be caused by inherited defects affecting immune system development, or they may result from secondary effects of other diseases (e.g., infection, malnutrition, aging, immunosuppression, autoimmunity, or chemotherapy). Clinically, patients with immune deficiency present with increased susceptibility to infections as well as to certain forms of cancer. The type of infections in a given patient depends largely on the component of the immune system that is affected. Patients with defects in Ig, complement, or phagocytic cells typically suffer from recurrent infections with pyogenic bacteria, whereas those with defects in cell-mediated immunity are prone to infections caused by viruses, fungi, and intracellular bacteria.