SEOUL—Authorities in South Korea are scrambling to contain an outbreak of the deadly Middle Eastern respiratory syndrome (MERS). At least 25 people have been infected—including one patient who traveled to China—and two have died in what is already the biggest outbreak of MERS outside the Arabian Peninsula. Scientists are wondering how a single imported case could have led to so many secondary infections.

The outbreak started when a 68-year-old man who returned from a business trip to four Middle Eastern countries on 4 May fell ill a week later. He was treated at several clinics before being diagnosed with MERS on 20 May.

Several countries have seen such imported cases since the MERS virus was first discovered in 2012, but the disease has never spread to more than a few other people, and the general consensus has been that MERS does not spread easily from human to human, because it infects the lower respiratory tract, from which it can't easily reach other hosts.

But the Korean patient appears to have infected at least 22 family members, health care workers, and fellow patients at a hospital where he was treated from 15 May to 17 May. (The hospital's name has not been revealed.) No special precautions were taken during that time, because the patient had not yet been diagnosed.

The early phase of the disease, just after hospitalization and when symptoms are getting worse, is the time when patients tend to secrete the most virus, says Christian Drosten, a virologist at the University of Bonn in Germany. “We know from Saudi Arabia that the virus can be transmitted during this time if people aren’t careful,” he says. Yet in similar situations, hundreds of exposed contacts did not develop the disease, says Peter Ben Embarek, the point person on MERS at the World Health Organization. “Why does this happen in one situation and not the other?”

The simplest explanation for the "superspreading event," as scientists call this type of spread, is a lapse in infection control measures at the hospital, Ben Embarek says. The SARS virus, which is distantly related to MERS, is known to have spread widely in 2003 when tubes were placed in patients' airways for mechanical ventilation, a procedure that can cause the virus to become aerosolized. Whether Korea's first patient was intubated is unclear. “We don’t really know what happened during those 3 days,” Ben Embarek says.

Other explanations are being looked at as well. The patient could be carrying a slightly different strain of the virus, or Koreans may be more susceptible to the disease than other populations, Ben Embarek says.

One important piece of evidence will be the genetic sequence of the virus. Ben Embarek says Korea has agreed to share samples with several labs working on MERS, including at Hong Kong University (HKU) in China and Erasmus MC in Rotterdam, the Netherlands. “We hope to have sequence analysis very soon so we can see any recent changes,” Ben Embarek says. “I don’t know if the samples have left the country or not, but I know there is an agreement that it will happen.” (Marion Koopmans of Erasmus MC tells ScienceInsider she has not heard from Seoul yet; so does HKU's Malik Peiris. "We have indicated our willingness to help. Have not heard anything concrete yet," Peiris e-mailed today.)

So far, Korea has quarantined close to 700 people to stop the spread of the virus. But a 44-year-old man who fell ill after visiting his hospitalized relatives in Seoul ignored quarantine orders and flew to Hong Kong on 26 May, after which he traveled by bus to Huizhou in China's Guangdong province. Alerted by the Korean government that the man had been in close contact with a MERS patient, local health authorities found him and placed him in isolation in Huizhou Municipal Central Hospital on 27 May. He tested positive for MERS on 29 May.

Chinese authorities have quarantined 67 people believed to have had close contact with the patient and are searching for 10 more, according to a 2 June report from China's official Xinhua News Agency.

In Hong Kong, 18 "close contacts" of the Korean traveler are under quarantine as well, according to a 1 June press release from the Hong Kong Department of Health, and 27 "other contacts" are under medical surveillance. None of those quarantined or under surveillance in Hong Kong and China have showed any signs of illness so far.

With reporting by Dennis Normile in Tokyo.