In order to promote carcinogenesis multiple factors must be orchestrated. The alteration of the cellular genome after a carcinogenic exposure may result in malignancy if apoptosis is prevented and the immune surveillance fails to eliminate the transformed cell. Infectious agents may exert these properties and transform a host cell. Viruses associated with human cancer are known as 'tumor viruses'. Most of them are capable of integrating into the host genome and have the ability to immortalize the target cell in order to allow their own replication. The infected cell expresses the viral genes, which are able to induce cell growth, proliferation and prevent apoptosis. This review focuses on Epstein-Barr virus, human papilloma virus, hepatitis C virus, hepatitis B virus, human herpes virus 8 and human T-cell leukemia virus, since they have been already established as causative agents of human cancer. An understanding of the viral replication mechanism may provide new targets for the development of specified viral therapy that may have an impact not only on viral infections but in human cancer as well.