Days after Amir Attaran, public health and law professor at the University of Ottawa, delivered a blistering attack in the Harvard Public Health Review on the International Olympic Committee and the World Health Organization for being in “deep denial” over the dangers that the Zika virus poses to the Games this summer, one of them has responded.

On Thursday, WHO made a statement along with the Pan American Health Organization, laying out how athletes and visitors can protect themselves. Practicing safer sex and avoiding “impoverished and overcrowded” areas in cities and towns, were two of six recommendations listed by the organizations, which also advised the use of insect repellent and choosing air-conditioned accommodations.

Another piece of advice, which some would say is rather obvious, was a warning for pregnant women to avoid travel to regions that have ongoing Zika virus transmission issues.

See also: How to prepare for the Zika virus this summer

But the latest statement from WHO didn’t seem to satisfy Attaran, who warned that the Zika virus could spread globally if the Olympic Games weren't postponed, canceled or moved to another location.

The warning is even more chilling than the one that was released by the Centers for Disease Control and Prevention last month, when Anne Schuchat, principal deputy director, said everything surrounding the virus was “a bit scarier than we initially thought.”

“But for the Games, would anyone recommend sending an extra half a million visitors into Brazil right now?” wrote Attaran in the Harvard Public Health Review, where he listed five arguments in favor of canceling the Games.

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His final reason was that it is simply irresponsible to hold the Olympics, where those visiting may return home and spread the risk to the rest of the public if they become infected. Attaran criticized WHO at the time for not releasing an official statement on Zika and the Olympics, calling its lack of action, “deplorable and dangerous.”

“WHO’s hesitancy is reminiscent of its mistakes with Ebola, all over again,” Attaran wrote.

Zika fears have been festering for months as the virus has been linked to paralysis and severe birth defects in pregnant women’s developing babies. And, as Attaran said, “Zika infection is more dangerous, and Brazil’s outbreak more extensive, than scientists reckoned a short time ago.”

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His warning spread across social media this week, with reactions ranging from alarm to dismissal. Some pointed to an April study in BioMed Central (h/t Quartz), an online journal that publishes peer-reviewed research, which suggested low individual risk, and risk of 5.76 symptomatic cases per 100,000 individuals.

Here are the rest of Attaran’s arguments for canceling the Games:

1) Rio de Janeiro has the highest rate of suspected Zika cases — 26,000 — of any Brazilian state, and its Zika incidence rate of 157 per 100,000 is the fourth worst. “Or in other words: according to the Brazil’s official data, Rio isn't on the fringes of the outbreak, but inside its heart,” Attaran said.

No one knows if Zika will decline during Rio’s drier and cooler winter months of July to September, making the Games a little safer. The country saw a sharp sixfold gain in dengue cases in the first quarter of 2016, which came as Brazil saw its biggest mobilization in history to combat the disease. Attaran said this means the baseline of disease transmitted by the Aedes aegypti mosquito has already started the year high.

2) This is a new, different and vastly more dangerous strain of Zika. A study published in Nature this week showed that infecting mice with Zika made heads and entire bodies smaller. While the congenital defects linked to the disease are bad enough, the preliminary findings of the effects on the adult nervous system aren’t good. For example, Zika can increase the odds of Guillain-Barré disease, a disorder in which your nerves are attacked by your body’s immune system, 60-fold.

3) The 500,000 tourists expected to flock to Rio for the Games will speed up the spread of Zika. “All it takes is one infected traveler: indeed phylogenetic and molecular clock analyses establish that Brazil’s cataclysmic outbreak stems from a single viral introduction event likely between May and December 2013,” said Attaran.

4) A faster-spreading virus will make it harder to invent new technologies to combat it, as it steals away the time advantage needed to get the disease under control. Scientists from a number of top institutions, including Harvard University, announced earlier this week that they have created a cheap test that will identify the virus within a few hours. A vaccine, though, remains a year and a half away, according to the WHO.

Some also note that Brazil really is a total mess right now, not just battling Zika, but wrapped up in the worst economic crisis since the Great Depression, while President Dilma Rousseff moves ever closer to being impeached over illegally manipulating fiscal accounts.

A very public warning against Olympic visitors was made earlier this week by former Brazilian soccer star Rivaldo Ferreira. In an Instagram post he warned that things are “getting uglier in Brazil,” and tourists should stay away, citing the killing of a 17-year old girl last weekend, a poor state of public health care and the mess in Brazilian politics.

“Only God can change the situation of our Brazil,” he wrote.

The full Harvard paper can be found here.