Mycoplasmas are widespread in nature as conditional pathogen, which may be the unique prokaryote that can cohabit with eukaryote and interact permanently with mammalian cells. Mycoplasma infection can be detected in many tumor tissues, continuous infection of mycoplasma can lead to transformation of mammalian cells, up-regulating expression of oncogenes, and some biologic changes of tumor cells, suggesting association of mycoplasma infection with tumorigenesis.