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.