The first consistent observations that viruses could be associated with some types of cancer where made almost a century ago. Since then researchers have spent a great deal of effort to address the infectious origins of human cancer. As a result of these studies, a strong link between some viral agents and several human cancers has been established. Some viruses as the Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), human T-cell lymphotropic virus type I (HTLV-I), immunodeficiency virus type I (HIV-I) and several human papillomavirus types (including types 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 and 66) have been classified as group 1 carcinogens by the International Agency for Research in Cancer (IARC). Infection by these viruses constitutes a heavy burden for human populations as it accounts for almost 15% of all human malignancies. Furthermore, many other viral agents have been classified as possibly carcinogenic to humans and others have been occasionally found in human tumors suggesting that this figure may be an underestimation of virus involvement in the etiology of human cancer. Therefore, viral infection appears as one of the main preventable cancer risk factors. We summarize the current state of knowledge concerning virus-induced/associated cancers and discuss its significance in the context of human carcinogenesis. Prevention and control of infection by these agents could dramatically reduce the incidence of some prevalent cancers and, consequently, have a great impact on public health.