The status of the immune system affects and is affected by both cancer and chronic infection. Some molecular mechanisms of immunity are relevant to both disease states. The role of infectious agents in cancer is generally underappreciated. However, approximately 20% of human cancers are caused by infectious agents and as such they rank second only to tobacco as a potentially preventable cause in humans. Specific viruses, parasites, and bacteria have been linked to specific human cancers. The infectious ethology for these specific cancers provides opportunities for prevention and treatment. The mechanisms by which an infectious agent can cause a cancer to vary. Some infectious agents that cause persistent infection and chronic inflammation lead to the formation of reactive oxygen and nitrogen species by macrophages at the site of the infection leading in turn to cell damage and cellular proliferation.