The primary infection with Epstein-Barr virus in an immunocompetent individual leads to infectious mononucleosis with symptoms of diphtheroid angina, lymph node swelling in the neck and hepatosplenomegaly. The most common age of infection lies between 15 and 25 years. The illness can affect a number of organs simultaneously and thus requires interdisciplinary diagnostics. For differential diagnosis, a differential blood analysis and a EBV quick test are required. The presence of IgM antibodies demonstrates the presence of the infection. Ultrasound of the abdomen can be made to determine the involvement of additional organs. In most cases, recovery occurs without complications. Acute cases can usually be handled successfully with medication. If symptomatic treatment fails, pharyngeal airway obstruction is possible and a tonsillectomy may be necessary. Otherwise, surgical treatment is obsolete. Generally, the prognosis is good. Severe courses and complications are rare.