What is the primary cause of swimmer's itch?

Swimmer's itch, also known as cercarial dermatitis, is primarily caused by the penetration of cercariae, which are the larval forms of certain parasitic flatworms, into the skin. These larvae are released from infected snails in freshwater environments and can penetrate human skin, leading to an allergic reaction characterized by itchy rashes. The condition is not due to infection, but rather the immune response to the larvae.

The other options do not directly relate to the cause of swimmer's itch. Contaminated water sources can lead to other infections or diseases but are not the source of the parasites responsible for swimmer's itch. Inadequate swimming facilities and insect bites may cause discomfort or other health issues but do not involve the specific parastic lifecycle that leads to swimmer's itch. Thus, identifying cercarial dermatitis as the cause accurately reflects the biological mechanism behind this condition.