A Belgian sailor who won a bronze medal at the 2012 Olympics has become the first person to fall sick after racing on Rio's polluted Guanabara Bay.

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 believes Van Acker contracted a severe intestinal infection while training in Rio de Janeiro in July.

Ill: Belgium's Evi Van Acker competes during the Laser Radial women event in Rio; Van Acker reported feeling sick after races on Wednesday

Underperforming: Van Acker, a favorite to win a medal, has consistently had poor results while competing in Rio

'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 Wednesday and evaluated further by the Belgian medical team that night, World Sailing spokesman Darryl Seibel said. Seibel 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.

Van Acker had a 'serious gastrointestinal infection a few weeks ago,' the Belgian Olympic Committee said in a statement. 'She has not fully recovered. It makes it difficult for her to go through long periods of sustained effort.'

The committee said a physiologist is working with Van Acker between now and the next races Friday 'so she can get the most out of her energy reserves.'

A doll's head, and plastics among the trash and untreated sewerage in the waters of the Guanabara Bay at the beginning of the month

The main issue with Brazil's water is the favelas nearby, impoverished neighborhoods where trash removal and sanitation services don't exist, leading people to dump waste there

Thursday was a day off for the Laser Radial fleet.

Guanabara Bay, which is due to be used for Olympic events including sailing and swimming, is hugely polluted - containing everything from human waste to trash.

Body parts were even found lurking in its depths as authorities tried desperately to clean it up before events begin there in a few days' time.

Brazil had pledged to reduce pollution in the water by 80 per cent before the games began, and while it is cleaner now than when the country won the bid, it is nowhere near meeting that target.

Olympic officials have insisted that sailing on the sprawling bay is safe, and sailing officials have said competitors have taken precautions. Even Brazilian sailors have said there's no danger — at least for those who compete there regularly.

Despite initial claims from Olympic chiefs that the dive pool was turned green by the algae, they later suggested it could be down to a change in the water's alkalinity

Ambush: A soldier guarding the Olympics was killed on Wednesday when the car he was riding in was sprayed with bullets after mistakenly entering a drug lord-controlled slum

German sailor Erik Heil, however, was treated for several infections he said were caused by polluted water during a Rio test regatta a year ago. He sails in the 49er class in which the two-man crew is splashed the whole race. That class is also prone to capsizes. The 49er competition begins Friday.

Van Acker, a favorite to return to the podium in Rio, has had consistently weak performances. She was second and 12th on Monday, second and 29th Tuesday and then 16th and 15th in tough conditions on Wednesday. She's 10th overall with four races left before the medals race. Although that would get her into the medals race, she has 47 points, currently 26 points out of medals position.

Seibel said that every morning World Sailing's medical and technical officials evaluate the latest water quality testing data provided by the government to make certain conditions acceptable.

'The standard our team uses in assessing water quality is the World Health Organization standard for primary contact (even though sailing is classified as a secondary contact sport),' Seibel said. 'For every day of competition thus far, and in the lead-up to the games, the water quality has met this standard.'

The Rio Olympics have been far from smooth sailing.

Organizers have been left embarrassed by the diving and water polo pools turning green.

Olympic athletes have complained of stinging eyes because Rio organizers have ‘over-chlorinated’ the outside swimming pool in a bid to prevent it going green.

They say huge quantities of the chemical have been put into the water next to the diving pool which turned a bizarre murky colour overnight.

But now athletes say that the water is damaging their game - and even making them feel nauseous due to the high levels of chlorine used.

And on Wednesday, a soldier guarding the Olympics was killed and at least two of his comrades wounded when their car was sprayed with bullets after mistakenly entering a drug lord-controlled slum.