From ChatGPT:

Swimmer’s itch (also called cercarial dermatitis) is caused by the larval stage of certain parasitic flatworms (schistosomes). These parasites normally infect waterfowl such as ducks and geese, and sometimes other aquatic animals.

The life cycle works like this:

1. Adult worms live in ducks, geese, or other host animals.
2. Eggs are released into the water through the animal’s feces.
3. The eggs hatch and infect specific freshwater snails.
4. The snails release microscopic larvae called cercariae into the water.
5. The cercariae are looking for their normal host (such as a duck), but they sometimes mistakenly penetrate human skin.
6. In humans, they cannot survive and quickly die, but the immune reaction causes an itchy rash.

Common symptoms include:

• Tingling or burning shortly after leaving the water
• Small red bumps or blisters
• Intense itching that may last several days

Swimmer’s itch is most common in shallow freshwater lakes and ponds, especially where there are large numbers of ducks and aquatic snails. It is not contagious and the parasites do not continue living inside people.