Illustration: Peter Oumanski

In October, Saudi Arabia will host millions of travelers on the hajj, the annual pilgrimage to Islam's holy sites. The hajj carries deep meaning for those observant Muslims who undertake it, but it also carries risks that make epidemiologists blanch. Pilgrims sleep in shared tents and approach the crowded sites on foot, in debilitating heat. They come from all over the world, and whatever pathogens they encounter on the hajj will travel back with them to their home countries. In past seasons, the hajj has been shown to foster disease, from stomach flus to tuberculosis or meningitis.

The Saudi Arabian government has traditionally taken this threat quite seriously. Each year it builds a vast network of field hospitals to give aid to pilgrims. It refuses visas to travelers who have not had required vaccinations and makes public the outbreaks it learns about. This year, though, the Saudis have been strangely opaque about one particular risk—and it's a risk that has disease experts and public-health agencies looking to October with a great deal of concern. They wonder if this year's hajj might actually breed the next pandemic.

The reason is MERS: Middle East respiratory syndrome, a disease that has been simmering in the region for months. The virus is new, recorded in humans for the first time in mid-2012. It is dire, having killed more than half of those who contracted it. And it is mysterious, far more so than it should be—because Saudi Arabia, where the majority of cases have clustered, has been tight-lipped about the disease's spread, responding slowly to requests for information and preventing outside researchers from publishing their findings about the syndrome.

Even in the Internet age, when data sources like Twitter posts and Google search queries are supposed to tip us off to outbreaks as they happen, one restrictive government can still put the whole world in danger by clamming up.

That's because the most important factor in controlling epidemics isn't the quality of our medicine. It's the quality of our information.

The Wall of Silence ——————-

To understand why MERS is so troubling, look back to the beginning of 2003. For several months, public-health observers heard rumors of a serious respiratory illness in southern China. But when officials from the World Health Organization asked the Chinese government about it, they were told that the countryside was simply experiencing an outbreak of pneumonia.

The wall of silence around what came to be known as SARS (severe acute respiratory syndrome) cracked only by chance. An anonymous man in a chat room, describing himself as a teacher in Guangdong Province, made the acquaintance of a teacher in California. On February 9, 2003, he asked her if she had heard of the illness ravaging his city. She forwarded his message to an epidemiologist she knew, and on February 10 he posted it to ProMED, a listserv that disease experts use as an informal surveillance system.

That email was the world's only warning for what was to come.

By mid-March there were already 150 cases of the new disease in seven countries. SARS wound up sickening more than 8,000 people and killing almost 800 in just nine months. Luckily, the disease was quelled in China and Canada (where travelers from Hong Kong touched off an outbreak in Toronto) before it had a chance to evolve into a more efficiently spreading strain.

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even in the internet age ... one restrictive government can put the world at risk.

With more warning, SARS might not even have gained a foothold outside of China. In Canada the virus quickly infected 251 people, killing 43. By contrast, the US had time to write new quarantine regulations, which made a difference: America had just 27 SARS cases, with no deaths and no hospital spread.

To health authorities who lived through SARS, MERS feels unnervingly familiar. The two organisms are cousins: Both are coronaviruses, named for their crown-shaped profile visible with an electron microscope.

For this disease too, the first notice was a posting to ProMED—this time by a doctor working in Jeddah, Saudi Arabia, describing a patient who had died several months before. That September 2012 communiquè, which cost the doctor his job, helped physicians in London realize that a Qatari man they were treating was part of the same outbreak. From there, MERS unspooled. People also fell ill in the United Arab Emirates, France, Germany, Italy, and Tunisia.

But Saudi Arabia, home to the vast majority of confirmed cases, remained far from forthcoming about what it knew. Announcements from the Ministry of Health supplied little useful detail and discussed illnesses and deaths that happened some indeterminate time in the past—possibly days, possibly even weeks.

So far the number of MERS cases is just a fraction of the toll from SARS, but health officials fear that the real count could be higher. Especially worrisome is the death rate among the afflicted: While SARS has been estimated to kill roughly 10 percent of its victims, MERS so far has killed 56 percent.

No One Thought It Would Happen Again ————————————

Certainly censorship about the spread of disease is nothing new. The largest well-documented pandemic, the great flu of 1918, is called the Spanish Influenza in old accounts not because it started in Spain (it may have begun in Kansas) but because Spain, as a neutral nation during World War I, had no wartime curbs on news reports of deaths.

To this day, no one is sure how many people died in the 1918 flu; the best guess hovers around 50 million worldwide. Regardless, since the virus took 11 months to circle the planet, some of those millions might have lived had the later-infected countries been warned to prepare.

After SARS, no one thought that it would happen again. In 2005 the 194 nations that vote in WHO's governing body promised not to conceal outbreaks.

And beyond that promise, public-health researchers have believed that Internet chatter—patterns of online discussion about disease—would undercut any attempts at secrecy. But they've been disappointed to see that their web-scraping tools have picked up remarkably little from the Middle East: While Saudi residents certainly use the Internet, what they can access is stifled, and what they are willing to say appears muted.

Nearly 100 years after the great flu, it turns out that old-fashioned censorship can still stymie the world in its ability to prepare for a pandemic.

So what now? The behind-door seething may be having an effect. A WHO team was finally allowed into Saudi Arabia in June, and the Saudi government has announced limits on the number of visas it will issue for this year's hajj. Meanwhile, governments and transnational health agencies have already taken the steps that they can, warning hospitals and readying labs. With luck, the disease will stay contained: In July, WHO declined to elevate MERS to a "public health emergency of international concern.

But the organization warned it might change its mind later—and if it does, we should fear the worst, because our medical resources are few. At present there is no rapid-detection method, no vaccine, and no cure.

While we wait to see the full extent of MERS, the one thing the world can do is to relearn the lesson of SARS: Just as diseases will always cross borders, governments will always try to evade blame. That problem can't be solved with better devices or through a more sophisticated public-health dragnet.

The solution lies in something public health has failed to accomplish despite centuries of trying: persuading governments that transparency needs to trump concerns about their own reputations. Information can outrun our deadly new diseases, but only if it's allowed to spread.