Slightly more than 20% of the global cancer burden can presently be linked to infectious agents, including viruses, bacteria and parasites. This manuscript analyzes reasons for their relatively late discovery and highlights epidemiological observations that may point to an involvement of additional infectious agents in specific human cancers. Emphasis is placed on hematopoietic malignancies, breast and colorectal cancers, but also basal cell carcinomas of the skin and lung cancers in non-smokers.