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

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. A large, modern hospital owned by the Samsung conglomerate, the center 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, known as MERS, was treated on May 27, the Health Ministry said in a statement on 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.