In the months leading up to the Paris Games, one of the biggest questions wasn’t only which athletes would be competing, but whether the Seine river would end up being safe enough to host them for some of the swimming events.

As the Games progressed, speculation only intensified. In advance of the men’s and women’s individual triathlons on July 31, the swimming portions of practice sessions were canceled due to water quality concerns, and when the events finally did take place on July 31, athletes were…less than thrilled by the conditions. Then, news broke over the weekend that multiple competitors had fallen ill after swimming in the Seine.

On August 4, Belgium even pulled out of the mixed-relay event scheduled for the next day after 35-year-old Claire Michel came down with what the Belgian publication De Standaard reported was an E. coli infection. While it’s not clear if her illness is linked to any of the other cases (and if any of them were caused by swimming in dirty water), the development underscores the safety concerns athletes and public health experts have voiced ever since it was announced that some Olympic events would be held in the iconic Paris waterway.

“Swimming’s been banned in that river for about a hundred years because it has, notoriously, been dirty,” Bill Sullivan, PhD, a microbiologist and professor at the Indiana University School of Medicine, tells SELF. “There’s a lot of sewage that creeps into the river. And there’s a lot of erosion that occurs with the soil washing potential animal feces into the water as well.” Combined, those two factors can make for a lot of nasties—like E. coli, the parasites Cryptosporidium and Giardia, and a whole slew of other bugs, according to Dr. Sullivan.

And while you might think that the risk decreases after a downpour—fresh water is good, right?!—the opposite is actually true, according to Dr. Sullivan. Heavy rain is a really big risk factor, he says, especially in old cities like Paris that have more antiquated sewer systems, because the resulting flooding washes all the waste on land into the river. With that in mind, it’s worth noting that rain fell in the days preceding the triathlons, even at the opening ceremony.

However, athletes were able to swim in the Seine after last-minute test results were deemed “compliant” by World Triathlon, the sport’s governing body, according to NBC. Then the mixed-relay event scheduled for August 5 went ahead as planned when officials—including representatives for World Triathlon and the International Olympic Committee—met over the weekend to review the latest water quality tests, which showed the bacteria count to be within the necessary limits, as the Associated Press reported.

But even a stellar test result isn’t a conclusive guarantee of swimming safety: Tests really only target certain species of fecal bacteria, effectively using E. coli as a proxy for other bugs, Dr. Sullivan says. Under most circumstances, that’s “probably okay, but it’s not a 100% assurance that the water is free of some of these other pathogens as well.” Overall, he says, the whole swimming situation makes him “pretty nervous.”