Stay out of the sun to avoid skin cancer. Don't smoke to avoid lung cancer. Don't eat raw fish to avoid liver cancer? It sounds a little strange, but in fact, the liver flukes Clonorchis sinensis and Opisthorchis viverrini have been found to be carcinogenic, inducing a cancer of the bile ducts that is very often fatal. The life cycle of O. viverrini is very similar to C. sinensis - they first pass through snails, then fish, and then finally the adults inhabit mammals that like to eat fish, including cats, dogs, and humans. The region of Khon Kaen, in northeast Thailand, has the highest incidence of these bile-duct cancers in the world - and 70% of the people in this region are infected with O. viverrini. A recent study produced large libraries of sequences of the transcriptomes - the genes that are transcribed at various stages - of these two important parasites, thus it is hopeful that new drug targets can be identified.
You can read more about the fluke/cancer link here.