Abstract Viruses are important in cancer for three main reasons: as a cause of cancer—about 15% of the worldwide cancer burden is due to viruses; in understanding of the biology of cancer-through the discovery and characterization of oncogenes and tumour suppressor genes; and in the treatment of cancer—some viruses selectively replicate in and destroy proliferating cells, viruses as foreign antigens may aid the recognition of cancer cells by the host’s immune system (‘xenogenization’), and viruses can also be used as vectors for immunization and for gene therapy. Viral cancers are prevented by early screening for tumours, screening for the virus in order to prevent transmission, and immunization as in the cases of hepatitis B virus and human papilloma virus.