Several taxonomically distinct human pathogenic viruses capable of upholding persistent infections have been recognized as important carcinogens. Jointly they are characterized by going into decade-long interactions with host cells and or tissues. Tumours arise after a long latent period in a few infected individuals. The cellular changes necessary for malignancy are only in part directly or indirectly caused by virus-cell interactions. Cofactors are assumed to be involved. The different routes to malignancy reflect the distinct strategies of each virus in its interaction with the host, which for the upkeep of chronic infections requires a tight control of both virus and cell multiplication and the extent to which an immune response is provoked. The size of the virus-cancer problem and the possibility of prevention makes virology one of the most promising areas of cancer prevention on a global scale. A much wider use of the vaccination against hepatitis B, especially in children, is warranted in developed countries.