Certain parasitic worms that can live inside the human body can also raise the risk of developing some kinds of cancer. These organisms are not found in the United States, but they can be a concern for people who live in or travel to other parts of the world.
Opisthorchis viverrini and Clonorchis sinensis are liver flukes (a type of flatworm) that have been linked to increased risk of developing cancer of the bile ducts. The bile ducts are tubes that connect the liver to the intestines. These infections come from eating raw or undercooked freshwater fish. They occur mostly in East Asia and are rare in other parts of the world.
Schistosoma haematobium is a parasite found in the water of some countries in the Middle East, Africa, and Asia. Infection with this parasite (an illness called schistosomiasis) has been linked to bladder cancer. Possible links to other types of cancer are now being studied as well.