About 90 percent of people are infected at some time in their lives with Epstein-Barr virus (EBV), usually with no ill effects. But individuals with compromised immune systems, such as people with organ transplants or HIV infection, have a greater risk of cancer occurring because of this virus. . . . "About 20 percent of all human cancers are caused by infectious agents, where about 80 percent of these infections are viral."
From here (hat tip to Gene Expression).
HPV which causes cervical cancer is the best known case of viral infection based cancer.