Infections may be responsible for over 15% of all malignancies worldwide. Important mechanisms by which infectious agents may induce carcinogenesis include the production of chronic inflammation, the transformation of cells by insertion of oncogenes and inhibition of tumour suppressors, and the induction of immunosuppression. Common characteristics shared by infectious agents linked to malignancies are that they are persistent in the host, often highly prevalent in the host population and induce cancer after a long latency. The associations between a selection of infectious agents and malignancies are covered in detail.