Tumour virus epidemiology.

A viral etiology of cancer was first demonstrated in 1911 by Peyton Rous who injected cell-free filtrate from a chicken sarcoma into healthy chickens and found it induced a tumour. Since the discovery over 50 years ago of the Epstein-Barr virus as the cause of Burkitt lymphoma, seven other human viruses or groups of viruses-hepatitis B virus, hepatitis C virus, human immunodeficiency virus type 1, some human papillomaviruses, human T-cell lymphotropic virus type 1, Kaposi sarcoma-associated herpesvirus and Merkel cell polyomavirus-have been linked to human cancer. Collectively, these eight viruses cause over 20 different types of cancer and contribute to 10-12% of all cancer, with a greater burden in low-and middle-income countries. For many viruses, immunosuppression greatly increases the risks of persistent infection, development of chronic sequelae and cancer. Although several viruses share similar routes of transmission (especially sexual activity, injection drug use and mother-to-child transmission), the predominant route of transmission varies across viruses, and for the same virus can vary by geographical location. In general, vulnerable populations at the greatest risk for viral infections and their associated diseases include people, especially children, living in low-to middle-income countries, men who have sex with men, people who use injection drugs and female sex workers. This article is part of the themed issue 'Human oncogenic viruses'.