Infectious mononucleosis is a benign, self-limited, lymphoproliferative disease caused by Epstein-Barr virus (EBV or HHV-4), frequently manifesting in adolescents and young adults. Most of the infected individuals are asymptomatic, with the virus remaining latent in the patient for a lifetime. Primary transmission occurs through contact with an infected person, particularly by saliva particles, with an incubation period estimated to vary from 2 to 5 weeks. Symptomatic patients often present a clinical viral syndrome characterized by fever, pharyngitis, fatigue, and posterior cervical lymphadenopathy. Patients may also manifest with tonsillitis as bilateral painful swelling of the palatine tonsils, frequently covered by an overlying yellowish pseudomembrane. The presence of palatine petechiae and enanthem is a common finding. Rare clinical complications may occur, including headache, splenic rupture, hemolytic anemia, aplastic anemia, skin rash, nephritis, jaundice, otitis media, neurologic complications, and pneumonia, which are more frequently observed in immunosuppressed patients. Microscopically, infectious mononucleosis presents as a diffuse proliferation of small- to medium-sized lymphocytes, hyperplastic lymphoid follicles, and scattered large infected cells that are mononucleated or polynucleated with prominent nucleoli. Only supportive treatment is necessary, since infectious mononucleosis is a self-limiting condition in immunocompetent patients.