Mayor Bill de Blasio said Friday the city is “acting on the assumption” that it’s not a case of if — but when — a person in New York is diagnosed with the coronavirus.

“We in New York City, take this very, very seriously,” said de Blasio, providing a health preparedness update at the Office of Emergency Management headquarters in Brooklyn.

“There’s no known cases in New York City [but] we have seen a spread that is so rapid now … that we have to act on the assumption that there will unfortunately be cases sooner rather than later in New York City,” he said.

While there aren’t any cases in the Big Apple — so far there has been one confirmed US case near Seattle, Wash., and one announced Friday in Chicago — the mayor said the city is on high alert because it has the largest Chinese immigrant population in the United States and many have traveled to the affected city of Wuhan, China.

De Blasio said “more needs to be looked at” than who has returned from Wuhan in the last few days. “We’d be shocked” if “no one” gets the disease here, said de Blasio.

All city hospitals and health providers have been trained on how to spot symptoms, the mayor said.

City Health Commissioner Dr. Oxiris Barbot added, “We want New Yorkers to go on with their daily lives,” and she wished everyone a “happy Lunar New Year.”