A much bigger challenge than emergency response, they say, is how to stop MERS being transmitted from animals to people in the Middle East, where it is endemic in camels. “The focus on South Korea would be better directed towards Saudi Arabia,” says David Heymann, a researcher at the London School of Hygiene and Tropical Medicine and chair of Public Health England, to stop the cases that continue to spark new outbreaks at the source.

Since it was first detected in Saudi Arabia in 2012, MERS-CoV has infected around 1,200 people worldwide, roughly 450 of whom have died, according to the WHO. The virus is thought to originate in bats and to jump to humans through an intermediate animal, such as camels. It does not easily spread between people, partly because it infects deep areas of the lungs, and is not coughed out. Most of the human infections, however, were the result of human-to-human spread, which can occur in hospitals when certain medical procedures combine with poor infection control to disseminate the virus. The latest clusters began when a South Korean man returned to Seoul from the Middle East, and visited four health-care facilities before he was diagnosed.

There is always a chance that as the virus spreads, it could acquire mutations that allow it to spread more easily between humans. But on 6 June, the South Korean health ministry announced that it had sequenced the virus in the current outbreak and that it was almost identical to past sequences from the Middle East. On the same day, the Chinese Center for Disease Control and Prevention posted a separate sequence to the publicly available GenBank database, from a man infected in the South Korean outbreak who then travelled to China, where he felt ill. Christian Drosten, director of the Institute of Virology at the University of Bonn Medical Centre in Germany has analysed this sequence and says that it is shows only minor mutations compared with Middle Eastern strains, none in areas of the genome thought to influence infectiousness.

A stream of new cases in South Korea might create the impression that the disease is out of control. But all cases reported so far have clear transmission routes from the initial infection, says Ian Lipkin, an outbreak specialist at Columbia University in New York. The country is now intensively tracing and isolating the contacts of those infected, and implementing strict infection controls in hospitals. Were cases springing up outside of hospitals that would be cause for worry, but that is not happening, says Lipkin.

In the Middle East, however, the virus continues to jump from camels to humans leading to hospital outbreaks. Heymann, who in 2003 led the global effort to contain severe acute respiratory syndrome, or SARS, says that authorities in the Middle East should do more to investigate how people catch the virus from camels.

Such studies would involve investigating the recent activities of infected people, finding out, for instance, whether they had had contact with animal carcasses or bodily fluids, had consumed fluids such as camel milk or urine, or had been near bat colonies. “It’s frustrating that all cases from animal infections have not been properly investigated,” says Peter Ben Embarek, leader of the WHO’s MERS team at the agency’s headquarters in Geneva, Switzerland. One obstacle is cultural, in that Saudis tend to be averse to discussing what they consider private matters, he says.

The outbreak in South Korea will probably put pressure on Middle Eastern countries to accelerate research and control of MERS, says Drosten.

Another outstanding mystery is why human cases have not been detected in African countries with large camel populations: Somalia has 7 million camels, and Kenya 3 million, dwarfing Saudi Arabia’s population of 260,000. “MERS is circulating in camels in many parts of Africa,” says Ben Embarek, “so camel-wise, it’s the same picture as in the Middle East.” One possibility is that human cases are going undetected because of poor surveillance. Another possibility is that cases in Africa are less likely or less serious, because MERS tends to cause serious illness only in people who have diseases that result from modern lifestyles, such as diabetes, which are more common in Saudi Arabia.