The early discoveries of Gross (1951) of viral causation of murine leukemias, the subsequent demonstration of retroviruses as the causative factors and the presence of similar viruses particularly in lymphoproliferative disorders in cattle, cats, and various rodent species, underlined for decades the justification of attempts to search for similar agents in human cancers, subsequently identified as retroviruses. When Stehelin et al.(1976) proved the cellular origin of retroviral oncogenes, it appeared to be almost odd to search for agents who possess their own oncogenes and may be involved in human cancers. Yet, until today, with the exception of the human T-lymphotropic retrovirus type I as causative agent of adult T-cell leukemia, endemic in the coastal regions of southern Japan, different groups of viruses emerged as prime factors even for some common human cancers. Besides Hepatitis C, they all contain DNA as genetic material and belong to very different virus families. Roughly 15% of the global cancer burden can be linked to these infections. An additional 5% can be attributed to bacterial (Helicobacter pylori) and parasitic infections (Schistosoma, Opistorchis, Clonorchis).