The search for infectious causes of human cancers: where and why (Nobel lecture).

Slightly more than 20% of the global cancer burden can currently be linked to infectious agents, including viruses, bacteria, and parasites. In this Review the reasons for their relatively late discovery are analyzed, and epidemiological observations that may point to an involvement of additional infectious agents in specific human cancers are highlighted. Emphasis is placed on hematopoietic malignancies, breast and colorectal cancers, as well as basal cell carcinomas of the skin and lung cancers in nonsmokers.