Severe combined immunodeficiency disease (SCID) is a genetic disorder characterized by the lack of T-lymphocyte and B-lymphocyte function. Natural killer (NK) lymphocyte function may also be missing. Individuals affected by this disorder have frequent, often serious infections because their immune systems are not capable of fighting any infections or diseases. At one time, children born with this disorder lived their lives in complete isolation from the outside world—some even in plastic bubbles. Newer treatments that avoid such isolation include bone marrow transplantation and gene therapy.