Infectious mononucleosis in older adults. Infectious mononucleosis as a manifestation of primary Epstein-Barr virus infection occurs uncommonly in adults over age 40. While fever is almost universal, older patients with the disease often present without lymphadenopathy, pharyngitis, splenomegaly, lymphocytosis or atypical lymphocytes. Jaundice and hepatomegaly occur more commonly in older patients than in adolescents and create diagnostic confusion. Often, infectious mononucleosis in this age group is confused with lymphoma, leukemia or biliary obstruction, or is classified as "fever of unknown origin.". 