Infectious mononucleosis is caused by the Epstein-Barr virus (EBV), a member of the herpes group. Apparently, the reservoir of EBV is limited to humans.

EBV is spread by contact with oral secretions. It’s frequently transmitted from adults to infants and among young adults by kissing. It also can be transmitted during bone marrow transplantation and blood transfusion.

Although major complications are rare, mononucleosis may cause splenic rupture, aseptic meningitis, encephalitis, hemolytic anemia, pericarditis, and Guillain-Barré syndrome.

The patient’s history may reveal contact with a person who has infectious mononucleosis.

After an incubation period of about 30 to 50 days in adults, the patient may experience prodromal symptoms. He usually reports headache, malaise, profound fatigue, anorexia, myalgia and, possibly, abdominal discomfort. After 3 to 5 days he develops a sore throat, which he may describe as the worst he’s ever had, and dysphagia related to adenopathy. He’ll usually have a fever, typically with a late afternoon or evening peak of 101° to 102°F (38.3° to 38.9° C).

Your inspection commonly reveals exudative tonsillitis, pharyngitis and, sometimes, palatal petechiae, periorbital edema, maculopapular rash that resembles rubella, and jaundice.

On palpation, you’ll probably note that nodes are mildly tender. You’ll usually find cervical adenopathy with slight tenderness, but the patient may also have inguinal and axillary adenopathy. You may detect splenomegaly and, less commonly, hepatomegaly.

Auscultation of the chest is usually normal.