A 16-month study commissioned by The Associated Press warns Olympic athletes of the contaminated waterways surrounding Rio de Janeiro.
Tests indicated the water is contaminated with raw human sewage, along with dangerous viruses and bacteria.
In response to the findings, biomedical expert Valeria Harwood warned athletes and other travelers in Rio not to "put your head under water."
The most polluted portions are where the Olympic rowing competition will take place and the starting point for the sailing races.
The Associated Press study also found that anyone who ingests just three teaspoons of water is "almost certain" to be infected with viruses. Those viruses are most likely to cause stomach and respiratory illnesses and in some cases heart and brain inflammation.
The adenovirus was found in 90 percent of the test sites during the 16-month testing period.
"That's a very, very, very high percentage," said Dr. Harwood, chair of the Department of Integrative Biology at the University of South Florida. "Seeing that level of human pathogenic virus is pretty much unheard of in surface waters in the U.S. You would never, ever see these levels because we treat our waste water. You just would not see this."
Olympic officials told the Bleacher Report that athletes and travelers are safe in Rio, despite the AP study.
The Rio Games will kick off Wednesday with soccer matches.
More on the Olympics:
- South Korea's Olympic uniforms come with built-in Zika protection
- Australian Olympic Committee calls Olympic Village in Rio 'unlivable'
- Russia won't be banned from Rio Olympics after doping report
For more news, check out today's 60 Second Circa.