THIS summer, when the Olympic Games are held in Rio de Janeiro, marathon swimmers, sailors and triathletes will be asked to compete in the highly polluted waters of Guanabara Bay and off nearby Copacabana Beach. Any athlete coming into contact with these waters has a high probability of becoming ill. Members of the United States junior national rowing team and competitive sailors have become sick with diarrhea, vomiting and flulike symptoms after training and competing around Rio, and they experience only “incidental contact” with the water. Marathon swimmers and triathletes will ingest this water — and the consequences could be deadly.

I am a long-distance open-water swimmer. Before marathon swimming was an Olympic sport, I twice set the overall men’s and women’s record for swimming the English Channel, and I was the first person to swim the Bering Strait from the United States to the Soviet Union. Like all athletes, I overcame pain in pursuit of my goals.

The only time I failed was when I became gravely ill during a race in the Nile River. Although I didn’t know it at the time, I was already sick with dysentery from training in the polluted water. Swimming through sewage, rotting rats and dead dogs, I struggled to finish the race. I didn’t want to stop because I was representing the United States, but after 15 miles, I nearly passed out. In the emergency room I was told I was extremely dehydrated and that I could have died.

I am worried that the Olympic coaches and athletes heading to Brazil this year are not well informed about these risks. Raw sewage from the Rio metropolitan area’s 12 million people — enough to fill 480 Olympic-size swimming pools — flows into Guanabara Bay every day. And that water flows directly onto Copacabana Beach, the site of the marathon swimming competition.