SEOUL—It is not unusual this week to see children and adults wearing face masks here as they walk the streets and subway stations. People are taking this precaution due to an outbreak of Middle East respiratory s yndrome (MERS) that began in May after a man infected with the disease returned to Seoul from the Middle East, initiating the first outbreak beyond that region.

On the morning of June 9, South Korea's Ministry of Health and Welfare announced eight new cases of MERS, bringing the total in South Korea to 95. (At this article’s posting time it stands at 108.) Many questions—and fears—surround the virus. How contagious is it? Has it mutated? Will the caseload in Seoul continue to rise or will the contagion be controlled?

During a special session on MERS at the World Conference of Science Journalists—taking place in this city—two South Korean scientists shared what is known about the virus and its propagation to date. Although there is no vaccine or drug to fight the virus, and up to 40 percent of people infected by it die, the researchers had some promising news.

It is still unclear exactly how the virus spreads among people but it is common for many pathogens to mutate and become increasingly contagious. So far, genetic tests carried out by various institutions indicate that the virus has not mutated, said Kee-Jong Hong, a doctor at the Pasteur Institute Korea.* Still, a mutation could occur any time, making the virus more transmissible.

Nearly all outbreaks of the virus worldwide have occurred within hospitals. In the Korean event no household transmission has been documented, said Sung-Han Kim, a professor of i nfectious d iseases at the Asan Medical Center in Seoul. Apparently, the virus takes some time to replicate once it has infected an individual. About 25 percent of people infected never show symptoms, but for the rest, symptoms arise only after the virus has replicated extensively in their bodies, making them very ill and likely to seek treatment.

Although experts do not think the virus spreads through the air, infections in hospitals have occurred in patients that shared the same room or waiting room, even if they were up to two meters apart. Doctors suspect that the use of the same medical instruments on infected and uninfected patients might be responsible for these transmissions.

Kim noted that although the overall mortality rate of infected people is 30 to 40 percent, most deaths have been among patients who were already hospitalized with another illness that may have weakened them and made them more susceptible. The mortality rate among infected hospital caregivers (who had no other illness) is less than 10 percent.

If the virus is not airborne and is not contagious until a person shows symptoms, and is therefore likely to be under medical care, why have authorities in Seoul closed schools? And why do some city dwellers insist on using facemasks?

Kim said these measures are wise precautions precisely because researchers have not pinned down how the virus spreads. He advised that people with cold like symptoms should wear a facemask to prevent transmitting any infection at this point, be it a common cold or MERS. Although available data show that MERS infections in this region occur almost exclusively in hospitals, 2,474 schools, (including 22 universities) have been closed due to an abundance of caution.

As for the future, Hong predicted that cases and deaths from MERS in South Korea will decline in the next week or two.

*Editor's Note (6/12/15): This sentence was edited after posting to correct Kee-Jong Hong's title. The original version erroneously identified him as the director of the Pasteur Institute Korea.