What organism acts as the host for the pest that leads to swimmer's itch?

Swimmer's itch, also known as cercarial dermatitis, is caused by a parasitic flatworm whose life cycle involves several stages, including a definitive host and an intermediate host. In this case, the intermediate host is typically a species of freshwater snail. The parasite releases cercariae, which can penetrate human skin, causing irritation and an allergic reaction.

This relationship highlights the role of snails in the life cycle of the parasite that leads to swimmer's itch. They play a crucial part by harboring the larvae, which then enter the water and can affect humans who are swimming or wading in infested waters.

The other options do not serve as the hosts in the specific context of swimmer's itch. While fish and frogs are part of aquatic ecosystems, they are not involved in the transmission of the parasites causing swimmer's itch. Worms, while a possible source of confusion, do not directly relate to this particular issue.