Scientists have discovered a 'superbug' in the waters that will be used for Olympic sailing and wind surfing events in Rio 2016.
The KPC enzyme, normally found in hospitals, is resistant to most forms of treatment.
And this was not an isolated discovery. The Instituto Oswaldo Cruz found the same enzyme in various spots along the Carioca River, raising serious questions about the safety of the waters around the Olympic host city.
"The illnesses caused by these micro-organisms are the same as those caused by common bacteria, but they require stronger antibiotics and, sometimes, can require hospitalization," the study's coordinator, Ana Paula D'Alincourt Carvalho Assef, told the Associated Press.
"Since the super bacteria are resistant to the most modern medications, doctors need to rely on drugs that are rarely used because they are toxic to the organism."
Even if they don't immediately fall ill, those who come into contact with the bacteria run the risk of becoming carriers of the microorganism, the institute said.
"Carriers can take these resistant bacteria back to their own environments and to other people, resulting in a cycle of dissemination," said the institute, which is affiliated with Brazil's Health Ministry.
With some 70 percent of sewage in this city of 12 million going untreated - and flowing, raw, into rivers, onto beaches and into the Guanabara Bay - water quality has been a major worry ahead of the 2016 summer games. In their Olympic bid, organisers pledged to slash by 80 per cent the amount of sewage and garbage that's pumped into the bay daily, but critics insist little has been done.
Water quality tests still show sky-high levels of fecal matter throughout much of the bay, and authorities have a near-blanket standing recommendation against swimming on any of its beaches. Flamengo beach, where the super bacteria was discovered, is among the Guanabara Bay beaches considered unfit for swimming.
The beach, which is adjacent to the Gloria Marina, the starting point for the Olympic sailing events, is also to be the viewing area for the events.
Ben Remocker, a former member of Canada's Olympic sailing team who represents athletes in two sailing disciplines, called the findings "serious for our athletes."
"We're going to be troubled by this," he said, adding he didn't think the possible health risks would dissuade sailors from taking part in the games. "I think the sailors are probably going to cross their fingers they aren't going to get sick."
The super bacteria were discovered in three out of five samples taken from along the course of the Carioca River. While it's not entirely clear how the bacteria may have gotten into the river, the statement quotes Assef as saying that no bacteria was discovered at the headwaters.
"The first point in which we detect its presence was ... after the river passes through areas with homes and hospitals," she said.
Organisers of the Rio games declined to comment, saying they would have to look into the findings before responding.