Introduction:
Etiology of many cancers remains an area still insufficiently well known. It is estimated that approximately 20% of cancers can be linked to the presence of infectious agents, in particular viruses, although not only. The purpose of this publication is to present current knowledge on infectious agents as initiators of the oncogenesis.

Current state of knowledge:
Hitherto, the effect of several infectious agents, among others, HPV, HBV, HCV, EBV, Helicobacter pylori, and Opisthorchis viverrini, on malicious transformation of cells has been proved. The oncogenic properties of the particular viral proteins have been documented. It has been shown that the E6 and E7 proteins of HPV are associated with the deregulation of the cell cycle, playing an important role in the carcinogenesis. HBV infection induces the formation of oncogenic viral protein X, which has transactivation properties. The HCV core protein and p70 protein are considered to be a factor that can induce a malicious cell transformation. The EBNA-2 and LMP-1 proteins of EBV are also identified as the oncogenic factors. The risk of developing cancer increases in people infected with HIV because the virus impairs the immune response by attacking and destroying the CD4+ T-cells. Bacterial and parasitic infections also influence carcinogenesis. It has been shown that long-lasting infection of Helicobacter pylori or Chlamydia spp., can lead to a cancerous process. Furthermore, the infections caused by Opisthorchis viverrini can also initiate oncogenesis, as the metabolites produced by the flukes are thought to be responsible for the development of bile ducts cancer.

Conclusions:
Viruses, but also some bacterial microorganisms and even parasites, are considered as risk factors in the development of cancer. In particular, chronic infections pose a major threat in this context.