Fluke, a flatworm that lives on or in other animals. Some flukes live on a single host. Many of these live on the gills or skins of fish, while others live in frogs. Most flukes require two or more hosts to complete their life cycles. The adult stage of these flukes is spent in higher animals, including human beings. They infest such parts of the body as the lungs, liver, blood, heart, or intestines. Schistosomiasis is a blood disease caused by flukes.

The eggs laid by the adults are expelled with the body wastes of the host. If the eggs come into contact with water and hatch, the young flukes infest snails. Some flukes complete their life cycles by developing in animals that eat the snails. Other species leave the snails when partly developed, and then invade other animals through the skin or when swallowed with water. Still other species infest fish or crayfish after leaving the snails, and yet others gather on plants. These enter the bodies of animals that eat the plants.

Flukes that infest human beings are common in the Far East and in the tropics. None is found in North America. Dogs, cattle, sheep, and horses, however, are infested with flukes in North America.

Flukes belong to the class Trematoda of the phylum Platyhelminthes.