Over the past few summers, as millions of Muslims set off on the annual Hajj pilgrimage to Saudi Arabia’s Mecca, members of the health community have speculated about whether this will be the year when religion and a new virus collide for a big, deadly outbreak.

Recently, the bug in question has been MERS — which stands for Middle East Respiratory Syndrome. Discovered in Saudi Arabia in 2012, MERS mostly hangs around the Middle East.

Patients infected with the virus usually experience a fever, cough, chills, sore throat, and muscle pain. Within the first week, the disease can quickly develop into pneumonia, requiring ventilation and organ support. Up to 40 percent of the time, MERS infections result in death — specifically from high fevers and pneumonia.

Earlier this year, MERS made the leap to South Korea, triggering the largest-ever outbreak outside the Middle East. By the time it was over in July, the virus had infected 186 people and killed 36.

Now Saudi Arabia is experiencing a new rise in cases — and once again, public health officials are on guard with the upcoming Hajj in September. Here’s what you need to know about the disease and the chances of a big outbreak.

MERS is in the news again because of the upcoming Hajj pilgrimage

MERS hit the news earlier this year because of an outbreak that happened in South Korea (the red lines in the diagram below). While Korea’s outbreak is now over, there’s been a recent upturn in cases in Saudi Arabia (shown in dark blue).

The country’s ministry of health reported eight new infections today, bringing the total to 66 active cases.

As this diagram shows, MERS hasn’t left Saudi Arabia since it was first identified in 2012.

So any big gathering there, involving thousands of people from around the world who could potentially swap deadly microbes, will ring alarm bells for public health folks. And there is no larger gathering of humans there than the Hajj.

A MERS outbreak at the Hajj is unlikely — but not impossible

There are a few reasons fears about a deadly outbreak are overblown.
Heading up to this year’s Hajj, the MERS situation looks pretty familiar. This isn’t the first time there’s been a rapid upturn in cases in the Arabian Peninsula just before the Hajj. And as I’ll explain below, there’s no reason to believe the virus has mutated to become any more contagious than before.

What’s more, the virus has never been shown to spread for any sustained period of time outside of health-care facilities. It mainly spreads in hospitals — which was the case with the South Korea outbreak (there was no spillover into the community) and in the Saudi outbreak right now (it’s concentrated at the King Abdulaziz Medical Center in Riyadh).

There are a couple of reasons MERS seems to flourish in hospitals. Experts believe it takes prolonged exposure to MERS to sicken people, and poor ventilation and the lack of spatial separation in hospital rooms can expose patients to large amounts of the virus.

The disease seems to disproportionately strike the kinds of people who stay in hospitals: elderly folks or those who are already weakened by other conditions, such as diabetes or underlying respiratory problems. (This is probably a different group from those who are undertaking miles-long pilgrimages.)

Since MERS seems to infect the lower lungs and is not easily coughed out, experts think MERS-infected patients on hospital ventilators may also help spread the virus. “In these settings,” explained Declan Butler in Nature, “medical procedures on an undiagnosed patient, for example to aid breathing, can generate aerosols from the lungs that contaminate the area and infect people nearby with the virus.”

There’s no reason to believe this year will be any different from the last, though outbreaks can always take the world by surprise.

The South Korea outbreak was big — but the MERS virus didn’t do anything unusual

With such a big outbreak in South Korea recently, it’s reasonable to be concerned about the virus in Saudi Arabia. But there were particular conditions that helped the outbreak grow, and they help explain why an outbreak in a setting like the Hajj is unlikely:

1) The initial patient went undiagnosed for a week, and in that time visited four health-care facilities, spreading the virus around at a time when he was very sick (and therefore very infectious).

2) It appears there were also lapses in infection-control practices where the man was treated. As Kai Kupferschmidt at Science magazine explained, bad ventilation in patient zero’s hospital room may have helped spread the virus.

3) Culture may play a role in helping spread the disease. People tend to seek care at big, overcrowded hospitals, the New York Times reports, and then they fill hospital rooms up with family members: “Family members and outside caregivers commingle with the sick in crowded emergency wards. They often stay with the patients in their rooms and do much of the nursing work — wiping sweat, emptying bedpans, changing sheets and exposing themselves to infections.”

4) Another factor: South Korea was quick to report MERS cases to the World Health Organization — something that other Middle Eastern countries have not done in the past. So it’s possible the South Korean outbreak is actually comparable to previous outbreaks, but only looks larger due to more diligent data sharing.

5) The scariest possibility is that the MERS virus somehow mutated to become more infectious among humans — and that’s why the South Korea outbreak is so large. But so far, the research on the South Korea outbreak suggests that’s not the case.

Overall, the way MERS behaved in South Korea is actually consistent with how it has behaved in the past. According to the WHO, “There is no evidence to suggest sustained human-to-human transmission in communities and no evidence of airborne transmission.”

MERS is still fairly mysterious and a possible contender for an epidemic

Having said all that, MERS is still a relatively new disease with some pretty big question marks around it.

In the past three years, it has infected a small number of people — just over 1,000, killing more than 400. How the virus can spread and how it’ll mutate if and when it infects more and more people remain open questions.

When the MERS virus was first discovered, it frightened the public health community for a couple of key reasons. It’s very deadly, and a lot is still unknown about it. There’s no treatment or cure. And the countries that typically harbor the most cases — i.e., Saudi Arabia —haven’t always been good about sharing information or promptly reporting cases to the global health community.

All these concerns hold today. Researchers still don’t understand many of the basics about the virus. They don’t know precisely how it spreads. They haven’t identified its natural animal host. Though they suspect bats and camels carry the virus (with a strong case building for camels as the primary hosts), the exact mode of animal-to-human transmission still isn’t clear, meaning it could simply be exposure to an infected animal or something like drinking camel milk.

What’s more, they don’t exactly know how long people remain infectious, how severe MERS is, and how best to manage people who are sick. It’s also not clear how this virus might change if it spreads among large clusters of people. So MERS could theoretically become more infectious among humans, according to The Lancet, boosting the risk of a pandemic.

For now, the global health community will be watching the Hajj, worrying about whether this year will be different. This virus may take the world by surprise once again.

Or it could just as easily go away. Vox