Editor’s Note (2/4/20): Our partners at STAT have reported that the study discussed in this story was based on faulty information.

People showing no symptoms appear to be able to spread the novel coronavirus that has caused an outbreak in China and led world health authorities to declare a global emergency, researchers reported Thursday in the New England Journal of Medicine. If confirmed, the finding will make it much harder to contain the virus.

The case described—from Germany—could help resolve one of the major unknowns about the virus, which as of Thursday night had infected nearly 9,700 people in China and killed 213. About 100 more infections have been reported in 18 other countries, but no deaths.

Some viruses, including SARS, which is another coronavirus, can only be passed when a person is showing symptoms. Others, like the flu, can be spread a day or two before the onset of symptoms. If people are contagious before they become sick, they can be unknowingly spreading the virus as they go shopping or to work or to the movies. Trying to snuff out the virus in that case is a much more difficult task.

What’s also concerning is that the spread from an asymptomatic person appeared to lead to two generations of cases, meaning the person who contracted the virus then passed it on to others.

The infection described in the new paper involved a woman from Shanghai who traveled to Germany for a business trip from Jan. 19 to Jan. 22 and displayed no signs of the disease, which include cough and fever. She only became sick on her flight back to China, and was confirmed on Jan. 26 to have the virus, known provisionally as 2019-nCoV.

On Jan. 24, however, a 33-year-old German businessman who had had meetings with the woman on Jan. 20 and 21, developed a sore throat, chills, and muscle soreness, with a fever and cough arriving the following day. He began to feel better and returned to work Jan. 27.

After the woman was found to have the virus back in China, disease detectives went to work, getting in touch with people who had been in touch with the woman—including the German businessman, who by then had recovered and appeared healthy during an examination in Munich. Tests, however, showed he had the virus.

On Jan. 28, three coworkers of the businessman tested positive for the virus. Only one of these patients had contact with the woman from Shanghai; the other two only had contact with the German man.

All four patients in Germany were isolated in hospitals and have not shown any signs of severe illness.

“The fact that asymptomatic persons are potential sources of 2019-nCoV infection may warrant a reassessment of transmission dynamics of the current outbreak,” the experts wrote.

Germany is one of four countries, along with Vietnam, Japan, and, as of Thursday, the United States, to report limited local transmission of the virus outside of China. The large majority of cases outside China were identified in people who had picked up the virus in China and then traveled to other countries.

The increasing risk the virus seems to be posing to countries outside of China led the World Health Organization on Thursday to declare a public health emergency of international concern, or PHEIC.

Until now, there was some debate whether asymptomatic transmission of the coronavirus was possible. Health officials in China said last weekend that they had seen evidence of such spread, but U.S. health officials and leaders at the World Health Organization had repeatedly said that was a question they were still trying to answer.

Even if asymptomatic people can spread the virus, they may be less likely to than people who are sneezing and coughing—routes for the virus to jump from one person to another.

Helen Branswell contributed reporting.

Republished with permission from STAT. This article originally appeared on January 30 2020