1. In infected people, the adult fluke produces eggs, which pass out of the body in stool (feces).
2. The eggs are eaten by a snail. Inside the snail, the eggs hatch and release larvae (called miracidia), which enter the snail's intestine. The miracidia go through two stages, then develop into a form that has a tail and can swim in water (called cercariae).
3. The cercariae are released from the snail into the water.
4. The cercariae penetrate the skin of a freshwater or brackish-water fish and form cysts in the tissues of the fish.
5. People become infected if they eat raw, undercooked, or salt-cured fish that contain the cysts.
6. In the small intestine, the fluke larvae leave the cyst and attach to the wall of the small intestine.
7. There, they mature into adult flukes.
8. In addition to people, various mammals (such as cats and dogs) and birds that eat fish can be infected by Heterophyes heterophyes.
Image from the Centers for Disease Control and Prevention Image Library.