Complex sentence: Opisthorchiasis occurs when the bile ducts are infected by the trematode parasite "Opisthorchis viverrini", which usually results from ingestion of raw or undercooked fish.
Split into simpler sentences: Opisthorchiasis occurs when the bile ducts are infected by the trematode parasite "Opisthorchis viverrini". Infection usually results from ingestion of raw or undercooked fish.