As many as 20% of human tumors are caused by viruses.  Some such viruses that are commonly recognized include HPV, T-cell Leukemia virus type I, and hepatitis B.
Viral oncogenesis are most common with DNA and RNA tumor viruses, most frequently the retroviruses. There are two types of oncogenic retroviruses: acute transforming viruses and non-acute transforming viruses. Acute transforming viruses induce a rapid tumor growth since they carry viral oncogenes in their DNA/RNA to induce such growth. An example of an acute transforming virus is the Rous Sarcoma Virus (RSV) that carry the v-src oncogene. v-Src is part of the c-src, which is a cellular proto-oncogene that stimulates rapid cell growth and expansion. A non-acute transforming virus on the other hand induces a slow tumor growth, since it does not carry any viral oncogenes. It induces tumor growth by transcriptionally activating the proto-oncogenes particularly the long terminal repeat (LTR) in the proto-oncogenes.
Viral Oncogenesis through transformation can occur via 2 mechanisms:
# The tumor virus can introduce and express a "transforming" gene either through the integration of DNA or RNA into the host genome.
# The tumor virus can alter expression on preexisting genes of the host.
One or both of these mechanisms can occur in the same host cell.