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 is typically diagnosed through blood tests. Treatment options include bone marrow transplants, which can restore immune function, and gene therapy, which aims to correct the underlying genetic defect. Early diagnosis and intervention are critical for improving outcomes.