Sailing's governing body says Evi van Acker of Belgium reported feeling ill after competing in the Laser Radial class on polluted Guanabara Bay.
World Sailing spokesman Darryl Seibel says van Acker, the bronze medalist at London, was evaluated by the chief medical officer after her races on Wednesday, and then evaluated further by medical officials from the Belgian team that night.
It's not immediately clear whether the illness is because of the polluted water.
Seibel said Thursday that this appears to be an isolated case and van Acker is the only sailor who has reported feeling ill in the opening days of the regatta.
Van Acker, a favorite to return to the podium in Rio, has had consistently worse performances. She was second and 12th on Monday, second and 29th Tuesday and then 16th and 15th in tough conditions on Wednesday.
An independent study by The Associated Press has shown high levels of viruses and sometimes bacteria from human sewage in the water.