The International Agency for Research on
Cancer (IARC) has comprehensively assessed the human carcinogenicity of biological agents. Seven viruses including Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV),
Kaposi's sarcoma herpes virus (KSHV), human immunodeficiency virus, type-1 (HIV-1), human T cell lymphotrophic virus, type-1 (HTLV-1), and human papillomavirus (HPV) have been classified as Group 1 human
carcinogens by IARC. The conclusions are based on the findings of epidemiological and mechanistic studies. EBV, HPV, HTLV-1, and KSHV are direct
carcinogens; HBV and HCV are indirect
carcinogens through chronic
inflammation; HIV-1 is an indirect
carcinogen through immune suppression. Some viruses may cause more than one
cancer, while some
cancers may be caused by more than one virus. However, only a proportion of persons infected by these oncogenic viruses will develop specific
cancers. A series of studies have been carried out to assess the viral, host, and environmental cofactors of EBV-associated
nasopharyngeal carcinoma, HBV/HCV-associated
hepatocellular carcinoma, and HPV-associated cervical
carcinoma.
Persistent infection and high viral load are important risk predictors of these virus-caused
cancers. Risk calculators incorporating host and viral factors have also been developed for the prediction of long-term risk of
hepatocellular carcinoma. These risk calculators are useful for the triage and clinical management of infected patients. Both clinical trials and national programs of immunization or
antiviral therapy have demonstrated a significant reduction in the incidence of
cancers caused by HBV, HCV, and HPV. Future researches on gene-gene and gene-environment interaction of oncogenic viruses and human host are in urgent need.