With the MERS death toll climbing higher in South Korea, here's what scientists know about the virus, how it spreads, and whether it poses a global threat. (Jhaan Elker/The Washington Post)

It began with a single coughing man.

A South Korean businessman had just returned from a trip to the Middle East when he started to feel ill. Feverish and wheezing, he made his way to one hospital, then another, then a third, then a fourth. By the time doctors figured out what was ailing him — a rare disease called Middle East Respiratory Syndrome (MERS) — the man had left a trail of the virus across the country.

On Friday a 72-year-old patient died from the illness, the 11th death reported in less than a month. But the spread of the disease, which has infected 126 people since the outbreak began last month, seems to have leveled off, and South Korean public health officials are urging calm.

[PHOTOS: South Korea battles MERS]

Still, across the country Koreans remain wary of this new and little-understood illness. MERS first appeared in humans in 2012, and three years later scientists and public health officials are still sorting out where it came from and how it’s spread.

Here’s what we do know:

MERS affects the lower respiratory system. A type of coronavirus (a common form of respiratory virus characterized by a crown-like shape), it’s most comparable to SARS, the disease that caused hundreds of deaths in China and a few other countries between 2002 and 2003 but has since disappeared. Like SARS, MERS seems to be spread between humans through close contact, probably via droplets of spit or other “respiratory secretions” containing the virus that are breathed or coughed out by the infected person. But because the virus is so new, it’s still not clear how this works.

According to the Centers for Disease Control, MERS usually starts out looking like any old common cold, causing fever, coughs and shortness of breath. But in some patients it can lead to severe complications, ranging from nausea, vomiting and diarrhea to pneumonia and kidney failure. Just over a third of people reported with MERS have died, according to the World Health Organization. As of this March, the WHO reported 1082 cases of the disease and 439 deaths, the vast majority in Saudi Arabia. The outbreak in South Korea is the biggest to happen outside Saudi Arabia since MERS was first identified.

The virus likely jumped from camels to humans. The first (known) case of MERS was reported in 2012 in Saudi Arabia. At the time, scientists didn’t know where it came from, but gene sequencing and subsequent research have pinned blame on camels. A study of a domestic camels near Dubai just published in the CDC journal Emerging Infectious Diseases found that nearly every animal tested had MERS antibodies, indicating that they’d been infected with the disease, and that camels are most infectious before the age of four. The animals then pass the disease onto humans, though it’s not clear how.

MERS has no cure. There is no vaccine against MERS, and no available antiviral treatment. The advice on the CDC’s “prevention and treatment” page pretty much amounts to: “Don’t catch it.” With the ominous addendum that, if you should fall dangerously ill, care is available to support your vital functions. But MERS is typically life threatening only in people who have underlying medical conditions — the elderly, people with compromised immune systems, etc.

The South Korean outbreak may have been caused by a “superspreader.” MERS is notoriously bad at spreading, especially compared with other viral illnesses. Its reproduction number (the average number of people that one sick patient will infect) is thought to be at or below one, though research varies and some studies put it higher. By comparison, the reproduction number for Ebola is about two, for smallpox it’s five, and for measles it’s between 10 and 20.

But in South Korea, something strange happened. One infected man seems to have passed on the disease at least 20 others, according to NPR. The businessman who started it all may be what’s called a “superspreader,” someone who carries an exceptionally large amount of the virus in his lungs, German virologist Christian Drosten told NPR. It didn’t help that the businessman visited several hospitals before being diagnosed and quarantined — sick people are more vulnerable to contracting the disease.

[How one joke photo has become a symbol of South Korea’s fears about MERS]

If the businessman was a superspreader, it’s actually good news for this outbreak, because it means that the people who caught MERS from the businessmen are unlikely to spread it to others. If he wasn’t, then officials must consider a scarier alternative: The virus may have mutated to become more contagious. For now, that seems unlikely to have happened. Alison Clements-Hunt, a spokeswoman for the World Health Organization, told the New York Times that the virus pattern found in South Korea matched what had been studied elsewhere. It showed no mutations or changes that might affect how easily it spreads.

In its current form, it is extremely unlikely to cause a pandemic. So long as MERS has a reproduction number of less than one, it won’t infect enough people to replace itself. Instead, it’s likely to die out, just as SARS seems to have done. And data released Friday by the WHO makes it seem as though this outbreak has passed its peak — the number of new cases diagnosed each day has fallen continuously since June 4.

People are frustrated with the South Korean government’s response. Though this MERS outbreak is unlikely to become a public health disaster, it wasn’t a shining moment for the South Korean health system. Hospitals weren’t alerted to the possibility of MERS as soon as the first case was confirmed, and the government didn’t release the list of 24 facilities where cases had been identified until this Sunday — at which point dozens of people had already been infected while visiting various hospitals. Meanwhile, Peter Ben Embarek, the MERS point person for the WHO, told Science magazine that the “superspreading” nature of the outbreak is best explained by lapses in infection control measures at Korean hospitals.

[South Koreans blame government in spiraling MERS outbreak]

President Park Geun-hye’s weekly approval rating dropped six percentage points in the aftermath of the outbreak, according a recent poll from Gallup-Korea. In the past couple of weeks Park has announced a wave of measures to control the outbreak — closing schools, tracking the cell phones of people under quarantine, releasing the names of hospitals where infections were identified — but the steps came too late for some critics. Choi Chang-woo, head of the civic group Citizens’ Solidarity for Safe Society, told the New York Times that the government should have been more transparent.

“Now the government is belatedly trying to mend the fence after the cow had already been stolen,” he said.