Belgian sailor Evi Van Acker has a bacterial infection that her coach believes she picked up while training in Rio's polluted Guanabara Bay. The poor water quality in the Olympic host city was a big pre-Games concern.
Van Acker won bronze in the Laser Radial class at the London Olympics in 2012 and was expected to be among the medal contenders this time around.
But her performances so far have been some way short of her best after she reported feeling ill on Wednesday.
Her coach, Wil Van Bladel, told the Belgian VRT network on Thursday that his charge had probably picked up the infection during training in the bay last month.
"Doctors say this (illness) can seriously disrupt energy levels for three months. It became clear yesterday that she lacked energy during tough conditions."
He added that "the likelihood that she caught it here during contact with the water is very big."
A World Sailing federation spokesman said this appeared to be an isolated case and that Van Acker was the only sailor who had reported feeling ill in the opening days of Olympic sailing.
Water-based events have been under the microscope during the Games, with the rowing program cancelled twice due to adverse weather conditions and the mysterious case of the green pool, which has been seen in swimming and diving competitions.