Mononucleosis is an acute infectious disease caused by the Epstein-Barr virus (EBV), a member of the herpes group. It primarily affects young adults and children, but some cases are so mild that the infection is overlooked. Infectious mononucleosis characteristically produces fever, sore throat, and cervical lymphadenopathy. It may also cause hepatic dysfunction, increased lymphocyte and monocyte counts, and development and persistence of heterophile antibodies. The prognosis is excellent, and major complications are uncommon.

Circulating B cells spread the infection throughout the reticular endothelial system, which includes the liver, spleen, and peripheral lymph nodes. Infection of B lymphocytes produces a humoral and cellular response to the virus. The T-lymphocyte response is essential in controlling the infection because this response determines the clinical expression of viral infection. A rapid and efficient T-cell response results in control of the infection and lifelong suppression of EBV, whereas an ineffective T-cell response may lead to excessive and uncontrolled B-cell proliferation, resulting in B-lymphocyte malignancies.