The Epstein-Barr virus (EBV) is among the most widespread of human viruses. It causes several different diseases, such as acute infectious mononucleosis (IM), chronic active EBV-infection (cEBV), the x-linked lymphoproliferative syndrome (XLP), polyclonal and oligoclonal lymphomae in connection with immunologic disorders, as well as African Burkitt's lymphoma and nasopharyngeal carcinoma. Pathogenesis, clinical features and diagnosis are discussed. In this connection, special tests on the latency and multiplication of the Epstein-Barr virus are presented. --The Epstein-Barr virus persists in the parotid gland of healthy individuals and is secreted via saliva. It is transmitted by direct contact and responsible for infectious mononucleosis. For the first time we succeeded in proving the multiplication of the virus in the palatine tonsil during acute phase of infectious mononucleosis.