Opisthorchis viverrini is a human parasitic trematode widespread in Europe. It causes a disease called opistorchosis. The life cycle of this parasite is very complex and lasts for 20 years. Infection occurs by eating undercooked fish containing larvae. The eaten larva gets into the duodenum, where it uses the Vater’s ampulla to move to the bile ducts where it reaches sex maturity. Adult individuals are anchored in the bile or pancreatic ducts were they lay eggs. The host organism provides nutrients for the parasite and eliminates the products of its life processes. Parasites suppress the host’s immunity responses. In mild cases, the disease appears as dyspepsia, abdominal pain, diarrhea and constipation. During prolonged invasion of the parasites symptoms may be more severe: hepatomegaly and malnutrition may be present. In rare cases, invasion leads to cholangitis, cholecystitis and cholangiocarcinoma.