Tumor viruses can be found in both the RNA and DNA virus kingdoms. All RNA tumor viruses belong to the retrovirus family. Directly transforming Class I RNA tumor viruses carry cellular oncogenes, picked up by accidental recombination, and usually selected for secondary modifications and high tumorigenicity by the investigator. They are not known to play any role for tumor causation in nature. Class II or chronic RNA tumor viruses do not carry cell-derived oncogenes but they often act by proviral DNA insertion into the immediate neighborhood of a cellular oncogene. Feline, murine, and avian leukemia viruses belong to this category. The human adult T-cell leukemia virus, (HTLV-1) and bovine leukemia virus (BLV) act by expanding the preneoplastic cell population and thereby provides the soil for secondary, cellular changes.

The DNA tumor viruses belong to three very different categories, the papovaviruses, adenoviruses and herpesviruses. Inactivation of the Rb and the p53 pathway by the viral transforming proteins is a convergent feature of the papova- and the adenoviruses. Since all DNA tumor viruses kill their host cell following their entry into the lytic phase, transformation and tumorigenicity are entirely dependent on a non-lytic interaction.

Cells transformed by DNA tumor viruses depend on the continued expression of the virally encoded oncogene. They provide thereby a convenient target for the immune surveillance of the host. Depending on the epidemiological history of the virus in relation to its natural host species, the immune surveillance of the host and the strategy of viral latency and survival can evolve into a truly symbiotic relationship, as best illustrated by the Epstein-Barr virus (EBV). Tumor development occurs only as an accident at the level of the host (immunosuppression) or the cell (specific translocations or other genetic changes).

The list of human viruses presently known to cause or to contribute to tumor development comprise four DNA viruses, namely Epstein-Barr virus, certain human papilloma viruses subtypes, hepatitis B virus, and Kaposi sarcoma herpesvirus (HHV-8); and two RNA viruses, adult T-cell leukemia virus (HTLV-1) and hepatitis virus C.