Background: Infectious mononucleosis (IM) is typically caused by Epstein-Barr virus (EBV), but can also be caused by drugs and other pathogens, such as cytomegalovirus (CMV) and hepatitis B virus. It shows a wide range of clinical and laboratory characteristics, which are presumed to be dependent on the patient age during the primary infection. This report describes the clinical features of hospitalized adults who developed EBV- or CMV-induced IM.
Methods: he medical records of adult patients, diagnosed as EBV- or CMV-induced IM at the Hallym Medical Center and Dankook University Hospital, between January 1999 and July 2004, were retrospectively reviewed.
Results: The analysis included 23 patients, consisting of 16 with EBV-induced IM and 7 with CMV- induced IM. Many of these patients were hospitalized under the impressions of either acute pharyngitis, acute hepatitis, fever of unknown origin or a malignant lymphoma. The vast majority of patients initially demonstrated lymphocytosis, with atypical lymphocytes. While patients younger than 20 years of age, usually presented with the classic triad of symptoms; IM-fever, pharyngitis and lymphadenopathy; those over the age of 20 often presented without pharyngitis or lymphadenopathy. There were no significant differences in the laboratory findings between EBV- and CMI-induced IM. Compared with patients with EBV-induced IM, however, those with CMV-induced IM were more likely to have abdominal pain (12.5% vs. 57%, P=0.04) and nausea or vomiting (25% vs. 75%, P=0.07), but less likely to have pharyngitis (69% vs. 14%, P=0.03) and cervical lymphadenopathy (75% vs. 14%, P=0.01).
Conclusion: IM in adults, especially in those above 20 years of age or if induced by CMV, are characterized by the atypical clinical manifestations. A higher index of suspicion and more attention must be paid to reduce unnecessary diagnostic work-ups and management.

Keywords: Epstein-Barr virus, Cytomegalovirus, Infectious mononucleosis