Dr. Camilla Rothe was about to leave for dinner when the government laboratory called with the surprising test result. Positive. It was Jan. 27. She had just discovered Germany’s first case of the new coronavirus.

But the diagnosis made no sense. Her patient, a businessman from a nearby auto parts company, could have been infected by only one person: a colleague visiting from China. And that colleague should not have been contagious.

The visitor had seemed perfectly healthy during her stay in Germany. No coughing or sneezing, no signs of fatigue or fever during two days of long meetings. She told colleagues that she had started feeling ill after the flight back to China. Days later, she tested positive for the coronavirus.

Scientists at the time believed that only people with symptoms could spread the coronavirus.