Using genetic analysis, it’s possible to trace the lineage of the virus like a family tree with branches around the world. One analysis, from researchers at the Los Alamos National Laboratory, showed that one branch emerged directly from China, with U.S. cases concentrated in Washington state. But a second branch of the virus grew in Italy, and was then imported to New York, rapidly becoming more prevalent.

“We tested people all through February, but it turns out we weren’t testing the right people,” de Blasio said in an emailed statement. “It’s painful to think about how things might have been different had we been able to test someone returning from Europe a month sooner.”

Jay Butler, the CDC’s deputy director for infectious diseases, compared the city to dry kindling in a wildfire.

“New York City is a global destination and had the opportunity for multiple introductions of a virus,” Butler said in an interview. “Because of the amount of travel to New York, particularly related to the increase of disease in Europe, there were multiple sparks landing at once.”

2. “Ride the Subway, Take the Bus”
Wash your hands and cover your cough: That was the main advice given to New Yorkers early in the outbreak to prevent the virus’s spread.

“We want New Yorkers to go about their daily lives, ride the subway, take the bus, go see your neighbors,” New York City Health Commissioner Oxiris Barbot said on March 2.

The mayor gave the same guidance. “From what we do understand, you cannot contract it through casual contact, so the subway is not the issue,” de Blasio said March 3. Days later, he said, “It has to go from someone who is infected to another person directly into their mouth, their nose, or their eyes.”

Health experts have now almost totally reversed that understanding. The CDC told people to begin wearing face coverings in public on April 3, at which point there were more than 5,000 new cases a day in New York. Later that month, the agency updated guidance to say that touching contaminated surfaces didn’t appear to be the primary mode of transmission. There have since been multiple case studies showing clusters of transmission in offices, at churches and other high-density settings.