X-linked severe combined immunodeficiency, or X-SCID, is a chromosomal disorder in which the immune system lacks multiple protective cells that defend the body from disease. As of 2024, approximately one in 75,000 males have X-SCID. X-SCID is the most common type of SCID, which encompasses a range of disorders that all involve defects in immune cells that fight infections, leaving the individual susceptible to life-threatening diseases. X-SCID, which typically only affects males, arises due to a mutation in the interleukin 2 receptor gamma chain, or IL-2RG, gene on the X chromosome. IL-2RG aids certain immune cells to develop their protective functions, so a mutation in the receptor results in a dysfunctional immune system. Doctors most commonly use bone marrow transplants to treat X-SCID. By studying cases of X-SCID, researchers more clearly defined the role of lymphocytes in immune system development and overall disease protection. Unless detected and treated early, the defect in immune cells from X-SCID makes the individual prone to severe, recurrent infections, which are almost always fatal.

David Phillip Vetter was born with X-linked severe combined immunodeficiency disorder, or X-SCID, and spent most of his life in an isolated sterile bubble to stay alive. X-SCID is a genetic disorder that causes a deficiency of protective immune cells that fight infections, which increases susceptibility to disease. Vetter’s weakened immune system meant that he would die if he encountered any bacteria, viruses, or other germs, so his doctors placed him in a sterile isolator, a plastic bubble that prevented germs from entering. Vetter was thus a germ-free human, meaning he developed in the absence of germ exposure. He grew up in his sterile isolator, earning him the nickname “Bubble Boy,” and lived there until his death in 1984 at age twelve. Researchers frequently studied Vetter’s development because he was one of the first individuals with X-SCID to survive over a year after birth. As one of the first germ-free humans, Vetter provided an opportunity for researchers to study what happens when a human develops without germs and his cells helped them determine the genetic cause of X-SCID.