In the nasopharynx, infectious virus is actively replicated. In contrast, in B cells, the viral genome is present, but mature infectious virus particles are not produced. The B cell may harbor EBV for prolonged periods. One of the antibodies they produce, the heterophile antibody, is a useful diagnostic marker of infection. The most common clinical manifestation of primary EBV infection is infectious mononucleosis.
Patients usually present with fever, pharyngitis, and lymphadenopathy.
Hepatosplenomegaly, a palatal enanthema, periorbital edema, and jaundice are less common features. A maculopapular diffuse rash occurs in 10 percent of patients, particularly in patients who have been given ampicillin. Thrombocytopenia is also common, and hepatocellular enzymes are abnormal in about 90 percent of patients. Cryoproteins and antigranulocyte antibodies are frequently present but are usually of little clinical significance. Most cases of infectious mononucleosis resolve in one to three weeks, although malaise and fatigue occasionally persist for several weeks to months.