Several types of cancer have infectious origins. The H. pylori bacterium responsible for stomach ulcers also produces stomach cancers. Viruses responsible for hepatitis B and hepatitis C also cause liver cancer. The human papilloma virus causes the vast majority of cervical cancer.
Just last week, UCLA researchers announced they'd found significant differences in the relative abundance of particular bacterial species in the mouths of pancreatic cancer patients and healthy people. They said the altered quantities eventually might be the basis of screening tests for pancreatic cancer. There, too, researchers were unable to tell whether the altered bacteria levels were a cause or an effect of the cancer.