Tumor viruses are those viruses that are able to infect cells and cause changes within the cell's operating machinery such that the cell's ability to regulate its growth and division is destroyed and the cells become cancerous.

Human papillomavirus, hepatitis B, Epstein-Barr virus, human T-cell leukemia virus, SV-40, and Rous sarcoma virus are all tumor viruses.

The ability of the Rous sarcoma virus to cause sarcomas (cancers of connective tissue) has been known since 1911, when Peyton Rous demonstrated that a sarcoma material from chicken could be filtered and the filtered fluid was still capable of inducing the cancer. The virus was both the first oncogenic (cancer-causing) virus to be discovered and (although not known until much later) the first retrovirus to be discovered. Another, well-known example of a retrovirus is HIV.

There are some 90 types of human papillomavirus, based upon the genetic sequence of their genomes...