Viruses and cancer

About 15% of human cancers can be attributed to virus infection, and viruses are second only to tobacco as a risk factor for cancer. In the future, a major proportion of these infections may be preventable by immunization, significantly reducing the worldwide cancer burden. There are two major mechanisms by which oncogenic viruses induce tumours: indirect oncogenesis, the virus infects a progenitor of the clonal tumour cell population, and usually persists in the tumour cells; indirect oncogenesis occurs when the virus does not necessarily infect the tumour progenitor cell, but exerts an indirect effect on cell and tissue turnover or on the immune system, predisposing to tumour development.