Who’s afraid of a little bacteria? Not these swimmers in Paris.

But floating next to Notre Dame or the Eiffel Tower is less magical when it rains. It’s not even possible.

The day after the river opened to public swimming for the first time since 1923, officials hoisted a red flag and promptly closed it again. Water-quality tests showed rainfall upstream had led to a high concentration of bacteria.

Swimming resumed a few days later when the weather cleared, but then the city had to evacuate the swimming site near the Eiffel Tower. A lifeguard had fished out what was later identified by police as an animal lung of an as yet unspecified species.

In all, the three Seine swimming spots have been closed roughly half the time since the river was reopened July 5, leading at times to long lines when they are open, and some doubts as to whether doing the breaststroke in the Seine is a particularly good idea in the first place.

“It is a little tempting, but I’m scared of getting eczema," said Pauline Mussat.

The 32-year-old dentist had joined the crowds watching bathers take a dip near the picturesque Île Saint-Louis. “I think I’ll send Laurent first," she said with a smile, nodding toward the young man sitting next to her.

Local authorities invested more than $1 billion to clean up the river for the 2024 Olympics, when the Seine hosted several events. It wasn’t easy; swimming in the river had been banned for over a century because of growing traffic and worsening pollution.

In Paris, rainwater flows into the sewage system. During heavy cloudbursts, the system used to often reach capacity and overflow into the Seine, officials say.

To limit sewage pollution, authorities built a massive underground storage tank in the heart of the city to store water during storms. It is designed to hold as much as 13.2 million gallons, about 20 Olympic pools’ worth. Local government workers have also gone door-to-door to persuade thousands of homeowners to connect their wastewater pipes to the sewer system instead of flushing directly into the Seine or one of its tributaries, the Marne, as is sometimes still the case. About half of the plumbing issues have been fixed.

The upshot of cleanup efforts, which ramped up in recent years after decades of work, is that there has been a 10-fold increase in the numbers of fish species in the Seine, including a six-foot-long catfish. In January, researchers even found three rare species of freshwater mussels that are extremely sensitive to pollution.

Plenty of Parisians and tourists have decided the water is good enough for them, too.

“I have zero worries," Kathleen Lang, a 51-year-old Australian nurse on vacation in France, said on a recent Friday.

Lang waited patiently outside the swimming area near the Eiffel Tower. The site had opened late that day because of what officials tactfully called a “pollution cloud," moving through the river from cities upstream.

Eventually, Lang slipped into waters teeming with anchovy-sized fish, along with around 200 people.

The three free sites, open until Aug. 31 as part of the annual Paris Plages, have space for nearly 1,000 people. Weather and pollution permitting, they are open most of the day—except for the one near Île Saint-Louis, which is closed weekday afternoons for tourist boats. Showers and lockers are provided.

Swimmers float leisurely, chat in groups as they tread water—and a few even attempt laps. The only river traffic here is other people and the mandatory yellow floating devices. Lifeguards stand on newly built wooden docks, where some swimmers sunbathe. From time to time, lifeguards ask them to either go back in the water or leave to make room for those waiting outside in line.

Megyn Price, a 54-year-old American actress, had to reassure her daughter back home about any potential health risk before getting in the water. But she had no regrets.

“It’s just so beautiful to see the city from that angle, I thought I might cry," Price said after her swim.

Parisian Caroline Gastaud-Nucera had been dreaming of swimming in the Seine since she was a child, listening to her grandparents tell stories about bathing in the river.

“The water is astonishingly clear, I can see my feet," the 54-year-old lawyer said, looking down at her red-painted toe nails as the slow-moving Seine slipped by.

Water quality can vary significantly depending on the weather.

After heavy rainfall, sensors installed to test water quality in real time showed a high concentration of E. coli. Most strains of the bacteria are harmless, but a high concentration indicates possible fecal contamination, which makes a welcome home for norovirus and other illness-inducing viruses.

“It will never be like a municipal swimming pool," said Paris deputy mayor for sports Pierre Rabadan. “There are natural hazards that we’ll never be able to control."

While the new tank is working well, the main problem is cities upstream. Without their own tanks, some continue to discharge wastewater after rainfall, officials said, and it can take hours to pass through the capital.

“There will always be days when it rains and we can’t go swimming," said Marc Guillaume, the prefect of the Paris region. The city plans to allow river swimming every year, though the locations may change.

Some Parisians suspect they’ll never take the plunge.

“It’s already much cleaner, that’s for sure. But go swimming in it? No way," said Sylvie Boucher, a Paris-based engineer, as she watched people entering the water.

“I admire them, they’re very brave," she said, before adding she would recommend a heavy dose of antibiotics to anyone taking a dip.