The close association between tumor and virus was established in 1908 by Ellerman and Bang when they observed that chicken leukemia could be transmitted through noncellular extract from the afflicted fowl. As a result, ”tumor DNA” was successively identified in several other species, i. e., mouse, cat, monkey and human beings. In this report, we reviewed the relationship between virus and tumor, the criteria determining this relation, the virus-associated human tumors, the potential carcino-genic process of virus inside the cell, and the application of virus in the diagnosis and therapy of the tumor in human. We used Epstein-B arr virus as a model to explain the latent as well as acute infection in the carcinogenesis and the disease progression marker in human. We further extrapolated the observations from herpes-virus and retrovirus to interpret the interactions between the host genes and the viral genes in the conversion process from proto-oncogene to oncogene. Finally, we discussed the current status of application of viruses in tumor diagnosis and gene therapy. In conclusion, although virus has been closely associated with tumor, the consequential carcinogenic process between virus infection and tumor formation remains to be elucidated.