Clonorchis sinensis is a human liver fluke found predominantly throughout China, Taiwan, Korea, Vietnam, and Japan. Commonly referred to as the Chinese liver fluke or Oriental liver fluke, this parasitic flatworm primarily targets humans as a definitive host, but a number of other animals are susceptible as well, including cats, dogs, pigs, rodents, foxes, badgers, and minks.

Ironically, the human liver fluke does not reside in or upon its host's liver; rather, the parasite lives in the biliary (bile) ducts. Adult flukes generally measure 10 to 25 millimeters in length, and may live up to 10 years inside a host. The host is likely to be infected with numerous flukes, and cases are known where a single human host contained over twenty thousand of the parasites.

Like many other parasites, these trematodes have a three host life cycle involving two intermediate hosts and one definitive host. Fluke eggs are introduced into water by means of feces from an infected host. The eggs are consumed by snails, which serve as the first intermediate host. Inside the snails, the eggs can complete their gestation and proceed to hatch. Newly hatched worms, termed cercariae, leave the snail host to live inside fish, the second intermediate host. Burrowing their way through the flesh of a fish, cercariae encyst themselves inside the fish's muscle tissue. The cercariae lose their tails, undergo some additional growth changes, and are subsequently termed metacercariae. The metacercariae reside inside their fish hosts, awaiting introduction to a definitive host. A human (or another fish-eating animal) becomes the final host by eating the raw flesh of an infected fish, and a single infected fish may contain over a thousand of the encysted worms.

The human liver fluke is said to affect millions of people in southeastern Asia (30 million by one estimate), causing many deaths and adverse health conditions. Raw fish and pickled raw fish are common in the diets of that region, which explains such a high rate of infection. Though some medical treatments are available, the host may suffer permanent damage or death depending on the degree of infestation and length of time carrying the parasites.