Around the world, infection is one of the most important causes of cancer. Almost one in every five malignancies can be attributed to infectious agents. Among infection-related neoplasms, cancers of the stomach, liver and cervix uteri detain the highest incidence figures, and are known to be largely attributable to Helicobacter pylori, hepatitis B and C viruses, and human papilloma virus, respectively. Other infectious organisms can also cause cancer; these include the Epstein-Barr virus (nasopharyngeal carcinoma, and different types of lymphoma), Human herpes virus-8 (Kaposi's Sarcoma), human T-cell leukemia virus type I (leukaemia, lymphoma), liver flukes (cholangiocarcinoma) and schistosomiasis (bladder cancer). Infection with human immunodeficiency virus, although strongly associated with an excess of cancer incidence at many cancer sites, is probably not carcinogenic per se, but acts mainly via immunodeficiency. The burden of infection-related cancers is still underestimated worldwide, due to the use of conservative population prevalence and risk ratio estimates. Furthermore, associations with new infectious agents remain yet to be explored.