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. A number of infectious agents have been identified which either cause or contribute to specific human cancers. They include two members of the herpes virus family, Epstein–Barr virus and human herpesvirus type 8, high risk and low risk human papillomaviruses (HPV), Hepatitis B and C viruses, a recently identified human polyomavirus, Merkel cell polyomavirus, the human T-lymphotropic retrovirus type 1 (HTLV-1), and human immunodeficiency viruses (HIV) types 1 and 2. In addition, human endogenous retroviruses have been suspected to play a role in human cancers. Besides viruses, other pathogens have also been identified. They include the bacterium Helicobacter pylori, a major contributor to gastric cancer, and parasitic infections, here in particular Schistosoma hematobium, a major cause of bladder cancer in Egypt, and liver flukes.
Although we know that presently slightly more than 20% of the global cancer incidence is linked to infectious events, some epidemiological observations suggest that this percentage will increase in the future. The recognition that no cancer linked to infections develops without additional modifications within the host cell genome permits the speculation that even cancers with well established chromosomal modifications deserve a careful analysis for an additional involvement of infectious agents. Prime malignancies suggested here as candidates for potential links with infections are hematopoietic malignancies, particularly childhood lymphoblastic leukemias, Epstein–Barr virus-negative Hodgkin’s lymphomas, basal cell carcinomas of the skin, and breast, colorectal and a subgroup of lung cancers. Although still hypothetical, this proposal is accessible to experimental verification. Even if only one of these speculations turns out to be correct, this would have profound implications for the prevention, diagnosis and hopefully also for therapy of the respective malignancy.