Adenosine deaminase (ADA) deficiency is an inherited disorder that damages the immune system and causes severe combined immunodeficiency (SCID). Children with SCID lack virtually all immune protection from bacteria, viruses, and fungi. They are prone to repeated and persistent infections that can be very serious or life-threatening. These infections are often caused by “opportunistic” organisms that ordinarily do not cause illness in people with a normal immune system. Infants with SCID typically experience pneumonia, chronic diarrhea, and widespread skin rashes, and grow much more slowly than healthy children. Some also have neurological problems such as developmental delay, movement disorders, and hearing loss. If not treated in a way that restores immune function, children with SCID usually live only a year or two.