you are correct that viruses cause some cancers. The main viruses associated with human cancers are HPV, hepB , hepC, Epstein-Barr, and human T-lymphotropic virus. Experimental data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage. The first vaccine for cancer came out last year when Gardasil was released for cervical cancer, caused by HPV.