BOSTON — On the first Monday in March, Michel Vounatsos, chief executive of the drug company Biogen, appeared in good spirits. The company’s new Alzheimer’s drug was showing promise after years of setbacks. Revenues had never been higher.

Onstage at an elite health care conference in Boston, Mr. Vounatsos touted the drug’s “remarkable journey.” Asked if the coronavirus that was ravaging China would disrupt supply chains and upend the company’s big plans, Mr. Vounatsos said no.

“So far, so good,” he said.

But even as he spoke, the virus was already silently spreading among Biogen’s senior executives, who did not know they had been infected days earlier at the company’s annual leadership meeting.

Biogen employees, most feeling healthy, boarded planes full of passengers. They drove home to their families. And they carried the virus to at least six states, the District of Columbia and three countries, outstripping the ability of local public health officials to trace the spread.