Swimming in contaminated water can expose skin to parasites, often from bird droppings.
Spot it: A red, patchy, itchy rash with pinpoint marks develops within 48 hours. Subsequent exposure can bring on a more severe reaction, and blisters may develop.
Treat it: The rash will go away on its own, but symptoms can last for a week. Ease discomfort with calamine lotion, hydrocortisone creams, or antihistamines.
Prevent it: Though the rash can occur after swimming in any natural body of water, it is less common in the ocean and running water. Avoid lakes with large bird populations or stagnant shallows, especially after a warm or dry spell. Rinsing off after a swim can help too.