Swimming is second to walking as the most popular exercise in the United States with more than 368 million annual visits to swimming pools. When people swim in pools, water parks, spas, hot tubs, lakes, rivers, and the ocean, they share the same water. If someone is ill, he or she can contaminate the water for everyone who is swimming. Contaminated recreational water can cause a variety of illnesses such as diarrhea or skin, ear, eye, and upper respiratory infections.