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.