Severe combined immunodeficiency (SCID) is a rare genetic disorder that affects the development and function of infection-fighting immune cells. It is also known as the "bubble boy disease" because its victims are extremely vulnerable to infectious diseases and some of them, such as David Vetter, have become famous for living in a sterile environment. SCID involves defective antibody response due to either direct involvement with B lymphocytes or through improper B lymphocyte activation due to non-functional T-helper cells. SCID patients are usually affected by severe bacterial, viral, or fungal infections early in life and often present with interstitial lung disease, chronic diarrhea, and failure to thrive. Ear infections, recurrent Pneumocystis jirovecii pneumonia, and profuse oral candidiasis commonly occur.

SCID is caused by mutations in different genes involved in the development and function of immune cells. More than 80% of SCID infants do not have a family history of the condition. However, development of a newborn screening test has made it possible to detect SCID before symptoms appear, helping ensure that affected infants receive life-saving treatments.

The most common treatment for SCID is a stem cell transplant, also called a bone marrow transplant, which means the child receives stem cells from a donor. Gene therapy as a treatment for some types of SCID has shown promising results in clinical trials, but has some risks so is not yet widely used. Children with SCID usually are cared for by a medical team of several specialists, such as a pediatrician, immunologist, and infectious disease specialist.