Most cases of EBV infectious mononucleosis are subclinical, and the only manifestation of EBV infection is serological response to EBV surface proteins with EBV serological tests. Airway obstruction and central nervous system (CNS) mononucleosis are also responsible for increased morbidity with infectious mononucleosis. Selective immunodeficiency to EBV, which occurs in X-linked lymphoproliferative syndrome, may result in severe, prolonged, or even fatal infectious mononucleosis.