Severe combined immunodeficiency is a primary immune deficiency that causes problems with T cells and B cells, leading to frequent infections, skin infections, failure to grow, and fungal infections. It can be fatal if not treated early in an infant's life. The disorder is caused by genetic defects linked to the X chromosome or chromosomes 19 and 20, including defects in the ADA and IL2RG genes. Treatments include bone marrow transplants from identical donors, living in a sterile bubble, and gene therapy, though the latter is not always successful.