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).