Infections by viral pathogens are involved in the etiology of liver cancer (hepatitis B and hepatitis C viruses), uterine carcinoma (papillomaviruses), lymphomas (Epstein-Barr virus), a specific type of leukemia (HTLV-1), and possibly in other kinds of cancer. Helicobacter pylori infection very probably has a causative role in the etiology of stomach cancer. In worldwide terms, viral agents play a considerable part, globally estimated to account for 15% of all cancer (zur Hausen 1987). In contrast, its role in the United States or in the Federal Republic of Germany is considerably smaller, estimated to account for ca. 5% (Harvard Report on Cancer Prevention 1996).