MERS - Middle East Respiratory Syndrome - was first reported on the Arabian Peninsula in 2012. Since then, there have been 1,154 confirmed cases of MERS worldwide leading to 431 deaths.

More than 85 percent of the reported cases have been on the Arabian Peninsula, primarily in Saudi Arabia, with all cases outside of the region linked by travel back to there.

It's gotten out again, and this time it's threatening East Asia.

As of now, more than 1,300 people in South Korea have been placed under compulsory quarantine after a 68-year-old man took ill after returning from a business trip to four Middle Eastern countries in early May. Suffering from respiratory distress, he went to two out-patient clinics and two hospitals before being correctly diagnosed with MERS. In the process, health officials believe, he may have infected at least 22 other people, two of whom are now dead.

According to the latest assessments, at least 30 people in South Korea have come down with MERS, with two dead. With memories of the SARS (severe acute respiratory syndrome) outbreak of 10 years ago that killed more than 750, people are wearing surgical masks on the streets and more than 200 schools have been closed.

But quarantines don't work if they're not enforced. Health officials say one confirmed patient ignored medical advice and traveled to China.

MERS, like its cousin SARS, is a coronavirus, so called because of the crown-like spikes on its surface. Coronaviruses are common and most people contract one at some point in their life, suffering mild to moderate respiratory tract distress, as in the common cold.

Coronaviruses also infect animals, mostly one species at most or a group of closely related species, but not people and animals. SARS and MERS are the exceptions. According to the US Centers for Disease Control, SARS can also infect monkeys, Himalayan palm civets, cats, dogs and rodents. In addition to humans, MERS can infect camels and bats.

"Although the majority of human cases of MERS have been attributed to human-to-human infections, camels are likely to be a major reservoir host for MERS and an animal source of MERS infection in humans. However, the exact role of camels in transmission of the virus and the exact route(s) of transmission are unknown," says the WHO website.

The MERS virus doesn't pass easily from person to person unless there is close contact, such as nursing a sick patient without adequate protection. Clusters of cases have appeared in health clinic where infection control practices were suspect. That's why WHO has warned that because of the sheer number of hospitals and clinics the first patient visited and was examined at, more cases can be expected.

According to WHO, more than a third of patients diagnosed with MERS die. There is no vaccine or specific treatment, other than "supportive and based on the patient's clinical condition," that is, doing everything possible to help the patient battle the illness.

In the South Korean case involving the 68-year-old man, he never told doctors he had been exposed to MERS. He had also been exposed to family members, visitors, medical staff and other patients in the same room and ward. Some patients were infected after being exposed to the man for a little as five minutes, WHO reports. So far, two of those people have died.

And South Korea's Ministry of Health has confirmed that two of the most recent cases are "third generation", meaning from exposure to someone infected by the original patient. So the virus is apparently on the move.

It was one of those confirmed cases that, on May 26, traveled "against medical advice" to Guangdong, China (by bus), via Hong Kong (by airplane). "He was symptomatic at the time of travel," WHO reports. "On 29 May, China informed WHO that the patient, who was isolated at a Huizhou hospital, tested positive for the MERS coronavirus."

According to a study published this week in the medical journal Lancet, coronaviruses have high rates of mutation and a tendency to cross into new host species and while this is of concern, especially in light of how deadly MERS and SARS can be, MERS transmission is not efficient enough to have "pandemic potential", the study's authors suggest.

WHO said that "consistent application of adequate measures for infection prevention and control has halted other large clusters of cases associated health care facilities," implying that it can be done again in this case. And they have not recommended any special screening at points of entry or travel restrictions.

The first MERS case in the US was diagnosed in 2014 - it was a health worker who had travelled to Saudi Arabia, the CDC says.