Opisthorchiasis is a parasitic infection of the bile ducts caused by liver flukes of the genus Opisthorchis, acquired by eating raw or undercooked freshwater fish carrying the larval stage. Endemic in parts of Southeast Asia, particularly Thailand and Laos where dishes of raw fish are traditional, as well as regions of Russia and Eastern Europe, it can persist for decades. Chronic infection inflames the biliary tract and is classified as a cause of cholangiocarcinoma, bile duct cancer, making it one of the few foodborne parasites that is directly carcinogenic. Prevention is straightforward: thorough cooking of freshwater fish kills the larvae, as does proper freezing. The disease illustrates why food safety advice on raw fish must be geographically specific, since risks acceptable in marine sushi culture do not transfer to freshwater fish in endemic regions.