Seven viruses including the 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 the International Agency for Research on
Cancer (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; and 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, high viral load, and viral genotype are important risk predictors of these virus-caused
cancers. Risk calculators incorporating host and viral risk predictors have been developed for the prediction of long-term risk of
hepatocellular carcinoma,
nasopharyngeal carcinoma and
cervical cancer. These risk calculators are useful for the triage and clinical management of infected patients. Both clinical trials and national programs of immunization,
antiviral therapy and screening have demonstrated a significant reduction in the incidence of
cancers caused by HBV, HCV, and HPV. Future research on gene-gene and gene-environment interactions of oncogenic viruses and the human host using large-scale longitudinal studies with serial measurements of biosignatures are in urgent need.