RIO DE JANEIRO — A Belgian woman who won a 2012 bronze medal became the first Olympic competitor to fall ill after sailing the polluted waters of Guanabara Bay.

Evi Van Acker reported feeling sick after Wednesday’s races, the governing body World Sailing said. Her poor performances 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.”

The poor quality of Guanabara Bay was at the forefront of the buildup to the Olympics. An independent study by the Associated Press has shown high levels of viruses in the water and bacteria from human sewage.

THE IOC WILL investigate the incident of a Kenyan track and field coach who was sent home after posing as an athlete and giving a doping sample in the runner’s name.

The IOC said it set up a disciplinary commission to look into the matter.

RAFAEL NADAL pulled out of mixed doubles before playing his first-round match.

His match with Garbine Muguruza against Lucie Hradecka and Radek Stepanek would have been Nadal’s third of the day. He reached the quarterfinals in singles and the semifinals in men’s doubles earlier Thursday.

NAMIBIAN BOXER Jonas Junias lost his first bout four days after being arrested on suspicion of sexual assault in the athletes’ village.

Junias, 22, was the Namibian flag bearer at last week’s opening ceremony.

OLYMPIC ORGANIZERS said around 30 percent of volunteers aren’t turning up for work each day.

An IOC spokesman called volunteers “the backbone of the games” but many aren’t so committed to working for free.

JOHN COATES, an IOC vice president, said the Rio de Janeiro Olympics have been “the most difficult games we have ever encountered.”

Rio has been plagued by problems, including a shortage of food for fans in some venues, empty seats, water pollution and crime around the edges.