X-linked severe combined immunodeficiency (SCID) is an inherited disorder of the immune system that occurs almost exclusively in males. Boys with X-linked SCID are prone to recurrent and persistent infections caused by certain bacteria, viruses, and fungi. The organisms that cause these infections are described as opportunistic because they ordinarily do not cause illness in healthy people. However, in people with X-linked SCID they cause very serious or life-threatening illnesses. Many infants with X-linked SCID experience chronic diarrhea and skin rashes, and grow more slowly than other children. Without treatment, affected males usually do not live beyond infancy.