A disease of children and young adults, characterized by fever and enlarged lymph nodes and spleen. EB (Epstein-Barr) herpesvirus is the causative agent.

Onset of the disease is slow and nonspecific with variable fever and malaise; later, cervical lymph nodes enlarge, and in about 50% of cases the spleen also becomes enlarged. The disease lasts 4–20 days or longer. Epidemics are common in institutions where young people live. EB virus infections occurring in early childhood are usually asymptomatic. In later childhood and adolescence, the disease more often accompanies infection—although even at these ages inapparent infections are common.