An oncovirus or oncogenic virus is a virus that can cause cancer. This term originated from studies of acutely transforming retroviruses in the 1950–60s, when the term "oncornaviruses" was used to denote their RNA virus origin. With the letters "RNA" removed, it now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus". The vast majority of human and animal viruses do not cause cancer, probably because of longstanding co-evolution between the virus and its host. Oncoviruses have been important not only in epidemiology, but also in investigations of cell cycle control mechanisms such as the retinoblastoma protein. The World Health Organization's International Agency for Research on Cancer estimated that in 2002, infection caused 17.8% of human cancers, with 11.9% caused by one of seven viruses. A 2020 study of 2,658 samples from 38 different types of cancer found that 16% were associated with a virus. These cancers might be easily prevented through vaccination (e.g., papillomavirus vaccines), diagnosed with simple blood tests, and treated with less-toxic antiviral compounds. Causality Generally, tumor viruses cause little or no disease after infection in their hosts, or cause non-neoplastic diseases such as acute hepatitis for hepatitis B virus or mononucleosis for Epstein–Barr virus. A minority of persons (or animals) will go on to develop cancers after infection. This has complicated efforts to determine whether or not a given virus causes cancer. The well-known Koch's postulates, 19th-century constructs developed by Robert Koch to establish the likelihood that Bacillus anthracis will cause anthrax disease, are not applicable to viral diseases. Firstly, this is because viruses cannot truly be isolated in pure culture—even stringent isolation techniques cannot exclude undetected contaminating viruses with similar density characteristics, and viruses must be grown on cells. Secondly, asymptomatic virus infection and carriage is the norm for most tumor viruses, which violates Koch's third principle. Relman and Fredericks have described the difficulties in applying Koch's postulates to virus-induced cancers. Finally, the host restriction for human viruses makes it unethical to experimentally transmit a suspected cancer virus. Other measures, such as A. B. Hill's criteria, are more relevant to cancer virology but also have some limitations in determining causality. Tumor viruses come in a variety of forms: Viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the hepatitis C virus (HCV), can cause cancers, as can