"German sailor Erik Heil claims he has contracted an infection after competing in polluted waters which will host sailing at next year's Rio Olympics.
Recent water quality tests revealed drug-resistant bacteria in the bay.
"I have never in my life had infections on the legs," Heil said on a team blog.
"I assume I picked that up at the test regatta. The cause should be the Marina da Gloria where there is a constant flow of waste water from the city's hospitals."
Heil's claims come after South Korean windsurfer Wonwoo Cho was taken to hospital during the week-long test event, with his coach Danny Ok claiming the cause was "probably from the water" at Guanabara Bay.
It led to the International Sailing Federation (ISAF) threatening to move the sailing and windsurfing events to the Atlantic Ocean if the situation did not improve.
But in a statement following Heil's infection, it said: "While work remains to be done, ISAF, Rio 2016 and all relevant local authorities are confident that the venue will be ready to host 380 sailors during the Olympic Sailing Competition in one year's time."
Three of the courses earmarked for the Olympics are in Guanabara Bay and three are in the Atlantic, with up to 1,400 athletes set to compete in water sports at the Games.
Research has shown that "super-bacteria" found in the water is usually found in hospital waste and produce an enzyme, KPC, resistant to antibiotics.
Nearly 70% of sewage in Rio - a city of some 10 million people - is spilled raw into the waters of Guanabara Bay."