Severe combined immunodeficiency (SCID) can be caused by mutations in a variety of genes. Although the clinical presentation can vary depending on which gene carries a mutation, there are several common characteristics observed throughout the different forms of SCID. Patients will typically present in infancy with severe, persistent infections of bacterial, viral, fungal and/or protozoal origin. In addition, these individuals have poor wound healing and failure to thrive. In states where newborn screening includes the TREC test for poor maturation of T cells, presymptomatic infants may be identified. T-cell lymphopenia is common to almost all forms of SCID, while the presence/absence of B-cells and NK-cells varies and can be used to help determine appropriate genetic testing. The prevalence of SCID in the general population is approximately 1/50,000 live births, with males showing a higher prevalence as the most common form of SCID is X-linked.