South Korea’s outbreak of MERS, a disease first detected in Saudi Arabia in 2012, is the largest to date outside the Middle East, where the vast majority of the more than 440 deaths attributed to it have occurred.

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SEOUL, South Korea — Nine new cases of Middle East respiratory syndrome, or MERS, were discovered in South Korea on Saturday, five linked to transmissions at one of the country’s largest and best hospitals, as officials struggled to contain the virus that has infected 50 people, killing five.

The announcement of five new cases at Samsung Medical Center in southern Seoul meant that a substantial new source of infection had been identified in the capital, a city of 10 million. The center, a large, modern hospital owned by the Samsung conglomerate, is staffed by some of South Korea’s best-trained medical personnel.

The five infected people had all been treated in the hospital’s emergency room, where a patient with the disease was treated May 27, the Health Ministry said Saturday. The patient had earlier infected two other people in the emergency room, a doctor and a visitor.

South Korea’s outbreak of MERS, a disease first detected in Saudi Arabia in 2012, is the largest to date outside the Middle East, where the vast majority of the more than 440 deaths attributed to it have occurred. Of the 50 cases in South Korea, 33, including the first laboratory-confirmed case, were found among the patients, visitors and medical staff of a hospital south of Seoul.

On Friday, the government identified that hospital as St. Mary’s in the city of Pyeongtaek, saying it was trying to find everyone who had been there during a two-week period in May.

“Finding them and placing them under monitoring is a key to containing the spread” of the outbreak, Moon Hyung-pyo, the health minister, said at a news conference.

The decision to identify St. Mary’s, a 250-bed hospital that opened in February, was a reversal for the government, which had been criticized for withholding the names of the six hospitals to which MERS cases had been traced.

The government asked that anyone who had been at St. Mary’s from May 15-29, including patients, employees and visitors, come forward, and it set up a hotline for them to call.

The mortality rate for known MERS cases is more than 30 percent, although it is possible that low-grade cases have gone undetected, a factor that would reduce the mortality rate.

Current medical knowledge suggests it takes a considerable amount of close contact for one person to transmit the virus to another, said Gregory Gray, a professor of global health and infectious disease at the Global Health Institute and the School of Medicine at Duke University.

Still, fear of the virus has escalated in South Korea after it was revealed that there had been transmissions beyond the initial hospital. More than 1,160 kindergartens and schools were temporarily closed, most of them in Gyeonggi province, which includes Pyeongtaek.

Testing of a virus sample collected from a South Korean patient showed that it was not a mutation from the strain seen in the Middle East, a senior Health Ministry official, Kwon Jun-wook, said Saturday at a news briefing. One effect of a mutation could be to make the virus more contagious.

The first or so-called index patient in the South Korean outbreak, a 68-year-old man who had been traveling in the Middle East, was hospitalized at St. Mary’s from May 15 to 17. The hospital was closed May 29.

The authorities have acknowledged that initial quarantine efforts at St. Mary’s were insufficient. After the 68-year-old was confirmed as having the MERS virus May 20, health officials initially tried to quarantine only those patients who had shared his hospital room, as well as visitors and medical staff members who had been in contact with them.

But May 28, a patient who had been in another room on the same floor tested positive for MERS, leading officials to expand their quarantine efforts to everyone who had been on that floor. By then, two patients from that floor had moved to different hospitals, where they have infected at least 13 people, according to the government.

One of the two, a 35-year-old man, rode a bus to Seoul, where he checked into Samsung Medical Center on May 27. The authorities have not formally identified the hospital, but officials have privately confirmed widespread news reports that it was the Samsung facility.

The man infected a doctor and a visitor there before he was found to have MERS on May 30, officials said. The doctor, who is now under quarantine, told South Korean news outlets that no one had told him about a possible MERS case at his hospital until he developed symptoms and asked to be quarantined Sunday.

As of Friday, more than 1,820 people suspected of having been in contact with any of the confirmed cases were being monitored in state-run facilities or were under quarantine at home.