What causes swimmer's itch?

Swimmer's itch is caused by aquatic parasites (flatworms) that naturally cycle between two different hosts, most commonly snails and ducks.

Unfortunately, the parasite stages released from snails are unable to distinguish duck skin from human skin. When they accidentally penetrate into the skin of a sensitive individual the parasites are unable to pass through the epidermis. They soon die, causing an allergic reaction ending with the formation of a papule (i.e., a red, raised itchy spot).