Of all the coronavirus’s qualities, perhaps the most surprising has been that seemingly healthy people can spread it to others. This trait has made the virus difficult to contain, and continues to challenge efforts to identify and isolate infected people.

It begins with a mild fever and malaise, followed by a painful cough and shortness of breath. The infection prospers in crowds, spreading to people in close reach. Containing an outbreak requires contact tracing, as well as isolation and treatment of the sick for weeks or months.

This insidious disease has touched every part of the globe. It is tuberculosis, the biggest infectious-disease killer worldwide, claiming 1.5 million lives each year.

Researchers have long known that masks can prevent people from spreading airway germs to others — findings that have driven much of the conversation around these crucial accessories during the coronavirus pandemic.