Reports of dangerously high levels of viruses and bacteria in Rio de Janeiro’s waters have raised — and renewed — fears that athletes are at risk of getting sick at next summer’s Olympics.

Americans Kalyn Robinson and Chip Peterson can’t help but wonder if a potential lifetime of health problems all goes back to the open water races they swam in the 2007 Pan American Games in Rio, where the waters are teeming with human waste.

There was more concern when 13 U.S. athletes came down with stomach illnesses at the recent World Junior Rowing Championships. A team doctor said she suspected it was due to pollution in the lake that will be used for Olympic rowing and canoeing.

Medical officials point out that thousands of other athletes have competed in Rio’s waters without getting sick. And doctors for Robinson and Peterson aren’t able to say that swimming off Copacabana Beach caused or even contributed to their diarrhea, unexplained cramping and unbearable stomach pain that began two weeks after they competed there eight years ago.

The two may never know if there was any connection.

“That’s the million-dollar question,” said Robinson, who competed under her maiden name, Kalyn Keller.

The worry is still there.

“I do find the fact that both Kalyn and I were diagnosed with so similar of diseases suspicious,” Peterson said.

Robinson was eventually told she had Crohn’s Disease and forced to retire for swimming, while Peterson had to deal with colitis, a similar inflammatory bowel disease that led to his colon being removed in 2013.

An analysis by The Associated Press of the waters where events will be held next year during the Olympics — including essentially the same course that was used for open water swimming during the Pan Am Games — showed a serious health risk for athletes, with dangerously high levels of viruses and bacteria that also could have ramifications for those competing in triathlon and sailing.

Dr. Scott Snapper, a Harvard Medical School professor who directs inflammatory bowel disease research at two Boston-area hospitals and chairs the national scientific committee of the Crohn’s & Colitis Foundation of America, was intrigued by the cases involving Robinson and Peterson. He said they could be a big help in understanding what causes their diseases, with some theories pointing toward a genetic predisposition.

Snapper stressed that no one can say the Rio waters were the direct cause of Peterson and Robinson becoming ill, though it might have been a contributing factor in triggering the diseases in two people who already were at risk genetically.

“If they had not gotten it then, then maybe they would have gotten it six months later,” Snapper told the AP. “Maybe they would have gotten it after a case of food poisoning or coming in contact with sick kids.”

Even so, one of America’s most prominent swim coaches expressed serious concerns about Rio’s Olympic plans.

Bob Bowman, who works with 18-time gold medalist Michael Phelps and once coached Robinson, said the Olympic open water competition should be moved to a cleaner, safer venue, even if that deprives Rio of staging events against the backdrop of one of the world’s most famous beaches.

“I am extremely concerned that people are going to swim in that water,” Bowman said. “You can’t put the health of the athlete in jeopardy and just look the other way because you’ve got a nice venue.”

At the rowing regatta that featured more than 500 competitors and concluded last weekend, officials said the number of illnesses reported were about as expected for such an event.

U.S. rowing coach Susan Francia, a two-time Olympic gold medalist, told the AP that 13 athletes and four staff members — including herself — suffered various gastrointestinal symptoms during the team’s two weeks of training in Rio that concluded with the weekend competition at the Rodrigo de Freitas Lagoon.

Dr. Kathryn Ackerman, the U.S. team physician, said athletes from several other countries stayed in the same hotel as the Americans, but did not seem to get as sick. Officials did not rule out that the Americans could have gotten ill from food or drinking water.

“We’re not really sure,” Ackerman said. “My personal feeling is, I think it’s from the lake.”

A spokesman for the Rio organizing committee attributed the American team illnesses to “classic travel symptoms” and said an event doctor treated eight Americans, three Britons and three Australians for symptoms including diarrhea.

Bowman said he can’t believe it was simply a coincidence that both Robinson and Peterson — young, fit, with no history of inflammatory bowel disease in their families — were stricken at roughly the same time with such similar conditions.

Robinson’s illness deprived her of a chance to compete in the inaugural Olympic open water competition at Beijing. Peterson’s career was sidetracked by frequent trips to the hospital and forced breaks from training for some six years.

Only after his colon was removed did the symptoms finally cease.

“Personally, I would never go in those waters again,” Robinson said. “There are plenty of other wonderful bodies of water on this planet.”

Peterson still competes, and won a gold medal at the recent Pan Am Games in Toronto though he failed to qualify for the Rio Games. Keller quit the sport to focus on her health, keeping her condition in check with a rigid diet and lifestyle.

“It ended Kalyn’s career,” Bowman said. “There was no doubt she was going to win a medal in Beijing, but she had to stop.”

Looking back, Peterson doesn’t remember the water in Rio looking all that different in 2007 compared with other venues where he had competed, even though water pollution has long plagued Brazil’s urban areas. Most sewage isn’t collected, let alone treated. In Rio, the waste runs through open-air ditches to fetid streams and rivers that feed the Olympic water sites and blight the city’s picturesque beaches.

Keller has a different recollection.

“It was weird,” she said. “Whenever we did open water swims, we pretty much always saw marine life in the water. But I remember doing a practice swim in the ocean at Rio and not seeing a single thing. Nothing but these weird, glowing specks. I asked if they were like little fish, but they didn’t know what they were.”

The absence of fish doesn’t always signal contamination, and athletes weren’t warned about any unsafe conditions. Robinson’s focus quickly returned to the open water competition, a relatively new discipline finally getting its moment in the sun. She competed over 10 grueling kilometers (6.2 miles), finishing just off the podium in fourth place. Peterson won a silver medal in the men’s event.

“I didn’t think anything of it,” Robinson said, her voice growing softer, “until I started to get sick.”

Rio organizers insist the waters will be safe during the Olympics. But Peterson said moving the event must be a serious option if the water quality doesn’t improve.

“I’m sure that there are places away from metropolitan areas that are cleaner,” Peterson said. “There are certainly pros to having it in the iconic venue of Copacabana, but the health of athletes is paramount.”