Earlier this month, a group of Brazilian scientists detected a drug-resistant super bacteria in the waters off of the Rio de Janeiro beaches where 2016 Olympic swimming events will be held.

This finding came on the heels of an investigation by the Associated Press that found “dangerously high levels of viruses and bacteria from human sewage” in Olympic and Paralympic venues.

As various athletes have withdrawn from the Games over concerns about the Zika virus, a famed distance swimmer is warning athletes headed to Rio to compete in water sports to consider the risks they’ll face in the highly polluted waters.

“The truth is that all of the water in all of the venues is severely polluted,” long-distance swimmer and author of Swimming in the Sink, Lynne Cox tells PEOPLE. “There is raw sewage from millions of people who flush their toilets into the Guanabara Bay each day.”

Cox says she began following the issue after reading about two open-water swimmers who became seriously ill after swimming in the waters off of Rio during the 2007 Pan American Games.

“These two swimmers got infections that affected the rest of their lives in huge ways,” Cox says.

Within a few months of this race, swimmer Chip Peterson was diagnosed with ulcerative colitis, a chronic inflammatory bowel disease, according to ESPN. The disease is believed to be influenced by both environmental and genetic factors, so Peterson, who ultimately had to have his colon removed as a result of the disease, couldn’t say for sure the polluted waters had triggered it.

Then, Peterson’s teammate in the 2007 race, Kalyn Keller Robinson was diagnosed with Crohn’s disease, a related condition, and soon retired from swimming all together.

“When I saw what was happening in Rio, I felt I needed to say something because it’s really wrong to have the best athletes in the world having to compete in sewage I mean it just makes no sense at all,” Cox says.