MERS has been called "a threat to the entire world" by the director of the World Health Organization. The virus, which had previously just hung around Saudi Arabia, has recently made the leap to South Korea, triggering the largest-ever outbreak outside the Middle East. Since then, travelers to China and Thailand brought the virus to those countries for the first time.

In South Korea, 166 people have been infected, 24 have died, and thousands have been under quarantine or observation. As of mid-June the numbers of cases seemed to be leveling off, but the WHO warned that "outbreaks are unpredictable."

What made the South Korea situation so surprising is that MERS — which stands for Middle East Respiratory Syndrome — isn't supposed to spread easily among humans. Scientists had thought of it as a virus that primarily lives in animals and only seldom affects people. In the past, when MERS has made the leap to human populations, the disease hasn't gotten very far.



But as it turns out, there's a lot we still don't know about MERS. It was only discovered in 2012, and the countries that previously harbored the virus, like Saudi Arabia, haven't always been transparent about their findings. Experts still don't understand exactly how MERS is transmitted, which is worrisome. And the fact that the disease is a deadly respiratory infection, from the same family as SARS (which infected more than 8,000 people during a 2003 global pandemic), has health officials on guard.

Experts are watching MERS because it can be deadly



MERS was first discovered in a patient who died in June 2012 in Jeddah, Saudi Arabia. Since then, the virus has largely stayed in Saudi Arabia, where it's believed to live in bats and camels, only rarely sickening humans.

When the virus does infect people, though, it can kill. Patients 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.

In the past, MERS hasn't spread very easily

The good news, however, is that MERS historically hasn't spread far beyond hospitals and the households of those who are ill. One study tracked 26 MERS patients and found they only gave the virus to 4 percent of their household contacts. Another study found that the reproductive rate of MERS outside of hospitals was less than 1 — in other words, for every case that appears in the wider world, fewer than one other person catches it, on average.

But there's an exception for hospitals, where the virus can spread more easily. There are a couple of reasons for this. Experts believe it takes prolonged exposure to MERS to sicken people, and poor ventilation and 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 (though some younger folks have fallen ill from MERS, too).

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."

South Korea currently has the biggest MERS outbreak outside the Middle East

What's happening in South Korea has attracted international attention because it's the first time MERS has caused a big outbreak outside of Saudi Arabia. In May, a 68-year-old man who had been traveling throughout the Middle East returned to Seoul and promptly developed MERS symptoms. He was diagnosed with the virus on May 20.

Although single cases or small clusters outside of the Middle East have popped up before (the virus has been found in 25 countries), this time more than 150 people got infected.

What's more, a patient exposed to MERS in South Korea then traveled to China while sick. On May 29, China confirmed that the patient tested positive for MERS. On June 18, Thailand confirmed its first MERS care — a traveler from the Middle East.

Now these countries — and their neighbors — are on alert. "All outbreaks are unpredictable," the WHO said. "This is especially true for a comparatively new disease like MERS, where so much about its epidemiology, modes of transmission, natural history, and clinical features remains poorly understood."

How the South Korea outbreak got so big

People have brought MERS to other countries before, but these cases didn't typically spark big outbreaks. So why does South Korea have so many MERS patients? There are a few possible reasons here:

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 has been quick to report MERS cases to the WHO — 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 has somehow mutated to become more infectious among humans — and that's why the South Korea outbreak is so large. Experts aren't ruling out this possibility, although the research on this outbreak suggests that's not the case.



For now, it seems the disease hasn't spread beyond hospitals in South Korea — which is actually consistent with how MERS 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. However, MERS-CoV is a relatively new disease and information gaps are considerable."

Still, the virus has only been known to researchers for a few years. 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 this time and how it'll mutate if and when it infects more and more people remain open questions.

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

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. It's a respiratory virus, and the possibility of airborne transmission hasn't been ruled out. And the countries that typically harbor the most cases, like Saudi Arabia, haven't 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, 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.

Finally, there's still no specific treatment or cure for MERS.

To get a better understanding of the South Korea outbreak, the WHO is undertaking a joint mission with health officials in the country. Beyond that, the big event the global health community will be watching is September's hajj, the annual pilgrimage of roughly 3 million Muslims from all over the world to Mecca, Saudi Arabia. If MERS has mutated to become more infectious, the hajj could act as a super-spreader event.



The virus could also just as easily go away. Since MERS hasn't been shown to spread in the wider world, many researchers are skeptical that we'll see a big outbreak.



Using SARS as a comparison (the two are genetically similar, and MERS appears to behave similarly to SARS once it gets into the health system), infectious diseases researcher David Fisman said he expects the South Korean outbreak could soon end. "Just as SARS was imported to dozens of cities during the 2003 outbreak but only Toronto, Singapore, Hong Kong, and Hanoi had sustained chains of transmission, you've had MERS imported lots, but here we have that rare event that makes everyone say the sky is falling."

Watch: Bill Gates discusses why the world isn't ready for a pandemic

Correction: A previous version of this story misstated the country of origin for the MERS patient in Thailand.