Several kinds of viruses cause cancer in humans, accounting for 10-20% of cancer worldwide. Human cancer viruses include human papillomavirus, hepatitis B virus, Epstein-Barr virus, human T cell lymphotropic virus, and Kaposi's sarcoma-associated herpesvirus. Cancer viruses alter the machinery of the infected cell to promote their own survival and growth. In the process, they interfere with normal cellular control mechanisms, leading to abnormal growth, genetic alterations and malignancy. Studies of virus replication and neoplastic cell transformation are revealing mechanisms by which these viruses cause human cancer.