What factor contributes to swimmer's itch?

Swimmer's itch is primarily caused by a reaction to certain parasites known as schistosomes, which are often found in freshwater environments and can be transmitted by snails. When individuals swim in affected waters, these parasites can penetrate the skin, leading to the characteristic rash associated with swimmer's itch. The lifecycle of these parasites involves freshwater snails, which release the larval stages into the water. Consequently, the correct answer reflects the biological mechanism whereby these larvae, upon contact with human skin, can disrupt the skin barrier and elicit an allergic reaction, resulting in the itching and discomfort associated with this condition.

The other options do not directly contribute to swimmer's itch. For instance, excessive swimming in chlorinated pools does not expose individuals to the parasites responsible for swimmer's itch. Saltwater environments are typically not breeding grounds for the freshwater schistosomes, and infections from underwater fungi are unrelated to the specific parasitic reaction that defines swimmer’s itch.