How did one patient with the deadly Middle East respiratory syndrome (MERS) infect so many others? That is one of the mysteries at the heart of an explosive outbreak of the viral disease in Korea that has so far sickened 41 and killed four. Now, scientists have a hypothesis at last. According to JongKoo Lee, director of the Seoul National University Hospital and former head of the Korea Centers for Disease Control and Prevention (KCDC), bad ventilation in one of patient zero’s hospital rooms played an important role. The investigation is still ongoing, he cautions. “But that is our tentative conclusion.”

The initial patient, a 68-year-old man who had just returned from a business trip to the Middle East, was treated at several clinics before being diagnosed with MERS on 20 May—9 days after he fell ill. He was treated at St. Mary’s Hospital in Pyeongtaek, a city about 1 hour south of Seoul, from 15 May to 17 May. Most of the people sickened have been tied to the same hospital.

The room in which patient zero stayed was originally meant for six people (the standard in Korea), but it had recently been divided, Lee says. It had only one small window that remained closed during the day, and there was no ventilation. An air conditioning unit cycled the air in the room, Lee told ScienceInsider. “The air conditioner is working in this small room with the door closed, so we are speculating that there is a very high density of virus particles in the air.” He also says that MERS RNA was detected on the membrane of the room’s air conditioning unit.

None of the scientists leading the investigation could be reached for comment, and a spokesperson at KCDC acknowledged only that a “team consisting of experts in various fields is investigating the ventilation system of [that] hospital.” Peter Ben Embarek, the point person on MERS at the World Health Organization (WHO), says that he has also been informed about a lack of ventilation in the room. “It’s very difficult to draw any conclusions based on what little information we have. We will hopefully get a more detailed picture soon of that part of the investigation.”

Bad ventilation alone would hardly explain the catastrophic spread seen in the hospital, says Christian Drosten, a virologist at the University of Bonn in Germany. But if there was bad ventilation plus a patient shedding a greater than usual amount of the virus, it could make a difference. In a study of dozens of MERS patients in intensive care in Saudi Arabia, for example, some had a much higher virus load in their exhaled breath than others, he says. “This must have been such a patient.” In combination with the constant circulation of the air in the room, this could help explain the high number of infections, Drosten says.

“A highly infectious case, combined with poor infection control can easily lead to this kind of cluster,” says Mike Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, Twin Cities. That that combination happened in Seoul was just bad luck, he says. “This could happen just as well in New York or Berlin.” But the government in South Korea did botch the initial response, he says. “There was no reason to close those schools, for instance. And distributing patients over many hospitals probably helped the virus spread,” he says. “But I think they are starting to get it right.”

After refusing for days to name the hospital at the heart of the outbreak, the Korean government finally revealed at a press conference today that 30 of the 41 patients had been infected at St. Mary’s. The Ministry of Health also asked people to contact a hotline if they had been at the hospital in the 2 weeks after the index patient was admitted. Finding and isolating these people is key to stopping the outbreak, says health minister Moon Hyung-pyo. Already more than 1600 people suspected to have had contact with a sick person have been isolated.

Early today, WHO announced a joint mission with Korea “to gain information and review the situation,” including epidemiological patterns, virus characteristics, and clinical features. A team of seven or eight scientists led by Keiji Fukuda, assistant director-general for health security at WHO, should arrive in Seoul this weekend, Ben Embarek says. The mission is expected to last about a week.

Meanwhile, KCDC finished its sequencing of the MERS virus causing the current outbreak and just today shared it with the U.S. Centers for Disease Control and Prevention. A spokesperson says it will release the sequence publicly tomorrow.