Just over a week ago, Michael Burke, the chief executive of Louis Vuitton, stood in the darkened space in Paris where his show — the last of fashion month — was about to take place and noted that, despite all the anxiety over the new coronavirus, there had been very few cancellations. He pointed out that many of the hundreds of people in the room had been traveling together and in enclosed spaces for weeks.

“Do you know anyone who is sick?” he asked rhetorically. Everyone looked around and shrugged.

But Mr. Burke didn’t know that earlier that day, in a doctor’s office in another country, Nga Nguyen had coughed.

Ms. Nguyen, 27, is, effectively, fashion’s patient zero. She was a guest at the Gucci show in Milan and the Saint Laurent show in Paris, two of the ready-to-wear season’s most popular, heavily attended events — and the first fashion world denizen to be identified publicly as testing positive for the new coronavirus.