Cancer is the leading cause of human death worldwide. A significant portion of human cancer cases, an estimated 20%–30%, attributed to an infectious etiology. So far, seven human tumor viruses are known to be associated with development and pathogenesis of cancers. The viruses include Epstein-Barr virus (EBV), human T-cell leukemia virus 1 (HTLV-1), human papillomavirus (HPV), hepatitis B (HBV) and C viruses (HCV), human herpesvirus 8 (HHV-8) that is also called Kaposi’s sarcoma herpesvirus (KSHV), and Merkel cell polyomavirus (MCPyV). We aim to define the oncogenic mechanism and pathogenesis of diseases caused by these viruses, especially EBV,HPV and MCPyV.