Approximately 12%-16% of human cancers worldwide (of 20-30 types) are attributed to viruses. Another 4% of cancer cases are attributed to other infectious agents. There are nine known viral causes of cancer in humans:
* Human immunodeficiency virus type 1 (HIV-1),
* Human T-cell lymphotropic virus type 1 (HTLV-1),
* Epstein-Barr virus (EBV),
* Kaposi sarcoma-associated herpesvirus (KSHV),
* Merkel cell polyomavirus (MCV),
* Hepatitis B (Hep B),
* Hepatitis C (Hep C),
* Human Papillomaviruses (HPV) (some genital-mucosal types), and
* Cytomegalovirus, a common virus in the herpes family.
Three of these (Hep B, Hep C, and HPV), have vaccines available, and one (HIV-1) has a partially effective experimental vaccine available. There are no vaccines available for the other five.
About 90 percent of people are infected at some time in their lives with Epstein-Barr virus (EBV), usually with no ill effects. But individuals with compromised immune systems, such as people with organ transplants or HIV infection, have a greater risk of cancer occurring because of this virus.
Newborns with congenital cytomegalovirus, a common virus in the herpes family, have an increased risk of developing acute lymphocytic leukemia.
See here, here, here, and here (note that some of the text above is closely paraphrased from these sources).
In non-humans, there is one known kind of cancer, affecting the same tissues that M.S. does, in Tasmanian devils, that is directly transmitted by bites without an intervening viral agent. It first appeared around 1990 and has killed about 70% of Tasmanian devils. It could lead to the extinction of the species by 2040. My source for this was a Science News story in 2009, but my link to this source is dead.
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