Severe combined immunodeficiency (SCID) is a rare genetic disorder that affects the immune system, making it difficult for the body to fight off infections. Individuals with SCID have little to no functioning T cells and B cells, which are crucial for a healthy immune response. This condition can lead to severe and recurrent infections, often appearing in infancy. SCID can be caused by various genetic mutations, and it is typically diagnosed through blood tests that assess immune function. Treatment options include bone marrow transplantation, which can restore the immune system, and gene therapy, which aims to correct the underlying genetic defect. Early diagnosis and intervention are critical for improving outcomes.