Severe combined immunodeficiency (SCID) is a rare genetic disorder that affects the immune system. People with SCID are unable to fight off infections and diseases due to a lack of functional immune cells. The condition can be referred to by other names, including "bubble boy disease" or "immunodeficiency with lymphopenia". Another synonym is "combined T and B cell immunodeficiency", which highlights the lack of both T and B cells in those with SCID. Additionally, SCID can be referred to as "primary immunodeficiency with defective cell-mediated immunity". Despite the different names, all refer to the same condition that severely compromises the immune system.