A Belgian woman who won a bronze medal at the 2012 Olympics has become sick after racing on polluted Guanabara Bay, the first sailor to fall ill from the waters that were heavily scrutinised leading to the Rio Games.

Evi Van Acker reported feeling sick after Wednesday's races, the governing body World Sailing said. Her poor performances have put her at risk of missing out on a medal in the Laser Radial class.

Her coach told the Belgian VRT network he believes Van Acker contracted a severe intestinal infection while training in Rio de Janeiro in July.

"Evi caught a bacteria in early July that causes dysentery," coach Wil Van Bladel said. "Doctors say this can seriously disrupt energy levels for three months. It became clear yesterday that she lacked energy during tough conditions. She could not use full force for a top condition. ... The likelihood that she caught it here during contact with the water is very big."

Van Acker was evaluated by the chief medical officer after her races on Wednesday and evaluated further by the Belgian medical team that night, World Sailing spokesman Darryl Seibel said. He added 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.