Severe combined immunodeficiency (SCID) is a rare genetic disorder that affects the development of T cells and B cells, leading to a non-functional immune system. It is characterized by early-onset severe respiratory infections, failure to thrive, and an increased susceptibility to infections such as ear infections, pneumonia, oral thrush, and diarrhea. SCID can be classified into two types based on the presence or absence of T and B cells, with some forms also involving natural killer cells. The disease can be caused by genetic mutations in various genes, with X-linked Severe combined immunodeficiency (XSCID) and adenosine deaminase deficiency being common forms. SCID is considered the most severe form of primary immunodeficiencies, resulting in a compromised immune system that is almost absent, making individuals extremely vulnerable to infectious diseases.