Acute infectious mononucleosis (IM) is a common illness that presents to urgent care facilities. The presenting symptoms of acute IM can range from the obvious, such as persistent exudative tonsillitis and marked cervical lymphadenopathy, to the
more vague symptoms of fatigue and malaise. Although most cases of acute IM are either subclinical or require only supportive care, some cases can have severe and life-threatening complications. Arrival at the correct diagnosis can be complicated by the timing of the patient’s presentation, since the sensitivities and specificities of the various laboratory and diagnostic tests change as the illness evolves. Even those patients without any complications can have prolonged symptoms and difficulty returning to school, work, or physical activities.