Infectious mononucleosis, also known as Pfeiffer's isease, mono (in North America) and more commonly known as glandular fever in other English-speaking countries. It occurs most commonly in adolescents and young adults, where it is characterized by fever, sore throat, muscle soreness, and fatigue. Infectious mononucleosis typically produces a mild illness in small children, and is often asymptomatic. Mononucleosis is predominantly caused by the Epstein-Barr virus (EBV), which infects B cells (B-lymphocytes), producing a reactive lymphocytosis predominantly consisting of atypical lymphocytes, a specific type of T-cell that gives the disease its name.