The run-up to the Rio 2016 Olympics has been particularly bruising, with fears of the Zika virus, doping scandals, political instability in Brazil, high crime levels, infrastructure struggles, and the recently reignited debate over the city's water sanitation. Headlines over the past week or so have streamed with talk about how “just three teaspoons of water” containing Rio’s raw human sewage would be enough to make even the healthiest of athletes sick.

 The Associated Press (AP) reported this was also accompanied with advice from Dr Valerie Harwood, chair of the Department of Integrative Biology at the University of South Florida, to tourists and athletes: "Don't put your head under water."

But what exactly is the risk, and just how sick could you become? A rash and niggling sore throat, or a deathly blood-boiling, flesh-eating tropical disease?

The recent fears were prompted after the AP published their 16-month-long study on the waterways of Rio de Janeiro, where many Olympians – such as sailors, rowers, and swimmers – will be competing. Along with the athletes, they also looked to the seawaters in preparation for the hundreds of thousands of tourists who are expected to flock to the idyllic-looking beaches.

One of the most riddled areas was Rodrigo de Freitas Lagoon, where the Olympic rowing and sailing events will occur, according to the report. A sample taken there in March 2015 as part of the AP investigation found 1.73 billion adenoviruses per liter of water. For perspective, thousands per liter in California would be deemed as worrisome.

Adenoviruses are a wide bunch of viruses that can be responsible for a host of infections, including gastroenteritis, conjunctivitis, respiratory illnesses, cystitis, and skin rashes. Although the illnesses can be pretty unpleasant in the short-term, there have been very few reports of a healthy individual dying from an adenoviruses infection.

Most infections can clear up within a matter of days or weeks without treatment. However, a few studies have pointed out that in some cases, an adenovirus infection can cause long-term brain inflammation in rats. But chances are we’ve all come across adenoviruses in our life numerous times, which has most likely been fought off by our immune system, or has perhaps caused a stomach bug or a couple of days coughing away in bed.

Unfortunately, there is a whole host of other sewage-borne pathogens that, in theory, could cause some real harm. These include cholera, dysentery, hepatitis A, and typhoid – all of which can be life-threatening regardless of an individual's health. AP investigators also looked at Rio’s waters for these by measuring coliform bacteria levels. Since coliform bacteria is always found in the feces of warm-blooded animals, it provides a strong indication that nasty waterborne pathogens are lurking in significant numbers.

Samples of water taken in June 2016 from Copacabana beaches – the site where the marathon and triathlon swimming events take place – and Ipanema beaches were actually “extremely low” in coliform bacteria. In this seawater, they found just 31 to 85 fecal coliforms for every 100 milliliters. By California’s water standards, 400 fecal coliforms per 100 milliliter of seawater can be considered safe for swimming. They did note, however, that the Rio figure rose above the California limit five times during 13 months of testing, although they didn’t specify by how much.

The AP report went on to say that these beaches did contain an “alarming” amount of rotavirus. The water off Copacabana and Ipanema beaches contained 7.22 million and 32.7 million rotaviruses per liter, respectively.

Rotaviruses are the most common cause of severe gastrointestinal infections among children. The symptoms can include severe diarrhea, vomiting, abdominal discomfort, and fevers. In their most recent project, the World Health Organization estimates the rotaviruses killed around 215,000 children under five years old in 2013. In healthy adults, however, the infection can be mild and may not even cause symptoms.

In sum, those “three teaspoons of water” you read about could get you ill, but perhaps not as sick as the headlines suggest. As opposed to obscure tropical diseases, the real threat from the water seems to be the milder, yet highly troublesome, adenoviruses.

Speaking about these levels of adenoviruses, Dr Harwood told AP: "That's a very, very, very high percentage. Seeing that level of human pathogenic virus is pretty much unheard of in surface waters in the U.S. You would never, ever see these levels because we treat our waste water. You just would not see this."

She added that these levels could make you "violently ill" if you ingested the water through your mouth or nose.

Additionally, it’s been pointed out that foreign athletes and tourists, particularly those from outside South America, are less likely to have immunity against many of the pathogens, compared to Rio residents who have been surrounded by them from a young age.

Nevertheless, the real killers – cholera, dysentery, hepatitis A, and typhoid – are only occasionally and fractionally higher than levels found in other coastal waters in California, according to this report. Of course, that’s not to say there is no risk. However, the most likely result of swallowing a few sips of the water is you'll have a pretty severe upset stomach or a nasty fever, which is still far from ideal if you’re hoping to smash world records.