Three patients in New York were under observation Friday amid concerns they may have coronavirus, according to state health officials.

Mayor Bill de Blasio said the city was working with state officials, although there were no further details on where in New York the individuals were.

"This is obviously a global health challenge. This is a situation that is emerging very quickly," the mayor said at a news conference. "There's a lot that we still don't know, but we have to prepare to keep our people safe."

It was a show of force and pre-emptive concern from de Blasio and the city's top health officials on Friday as they warned New Yorkers about the dangers of coronavirus, a flu-like virus that has claimed at least 41 lives and sickened more than 1,200 people in China — mostly in the city of Wuhan — and abroad, forcing officials there to shut down some airports and ban travel.

"So we are preparing on the assumption that it's not a question of if, but a question of when, we start to have cases of coronavirus here in New York City," de Blasio said.

According to state Department of Health officials, three people were being isolated and under investigation Friday, their cases referred for testing at the Centers for Disease Control and Prevention (CDC). Two cases have been confirmed in Chicago and Washington state.

With New York City being home to the largest Chinese population outside of China, and as travel increases ahead of Lunar New Year celebrations, officials asked New Yorkers to be extra cautious.

"We are asking New Yorkers to do is that is if they have traveled to Wuhan in the last 14 days, and they find themselves with symptoms of fever and a cough or shortness of breath, that they contact their provider and share this information," said Dr. Oxiris Barbot, the city's health commissioner.

Symptoms of the virus include:

Runny nose

Headache

Cough

Sore throat

Fever

The virus can also cause wheezing and pneumonia. It is a member of the coronavirus family that's a close cousin to the deadly SARS and MERS viruses that have caused outbreaks in the past.

"Our advice to New Yorkers during this time is to go about their daily lives, but to practice everyday precautions during flu season," Barbot said. "That includes covering their mouth and their nose when they cough or they sneeze, to wash their hands often and to stay home if they are not feeling well. Additionally, it is not too late to get the influenza vaccine."

Health officials have yet to fully understand how destructive the virus could be, or exactly how quickly it can spread. For now, officials here at home are preparing for what they say is the certain arrival of the virus to the shores of New York.

The Centers for Disease Control and Prevention is expecting more Americans to be diagnosed with the newly discovered virus in coming days, as worldwide the number of confirmed cases has passed 1,200.

Still, "CDC believes that the immediate risk to the American public continues to be low at this time, but the situation continues to evolve rapidly," said CDC's Dr. Nancy Messonnier.

The incubation period is thought to be two weeks. But it's also a heavy flu season, and some of the symptoms are similar, Messonnier said.

"We want everyone to err on the side of caution if they have those symptoms and have a travel history," she stressed.

A Chicago woman became the second U.S. patient diagnosed with the dangerous new virus from China, health officials announced Friday.

The woman in her 60s returned from China on January 13 without showing any signs of illness, but three or four days later she called her doctor to report feeling sick.

The patient is doing well and remains hospitalized "primarily for infection control," said Dr. Allison Arwady, Chicago's public health commissioner.

Earlier this week, a man in his 30s in Washington state became the first U.S. patient, also diagnosed after returning from a trip to the outbreak's epicenter in central China.

Officials said the first two patients did the right thing, recognizing their travel risk and alerting doctors to it, so that health authorities could get them isolated to prevent spread, tested promptly, and start monitoring people who've had close contact with them.

It took about 36 hours for the first U.S. patient to be tested, get the results from the CDC in Atlanta and be hospitalized after he went to a clinic Sunday morning, according to a timeline released Friday by health officials in Washington state.

Illinois health officials did not identify the hospital where the woman is in isolation.

The Washington state patient was in satisfactory condition Friday in an isolation unit at Providence Regional Medical Center in Everett, 30 miles north of Seattle. Nurses who've trained for Ebola patients were providing his care, wearing hoods with plastic face shields and using respirators to breathe filtered air. They were bagging and storing disposable gloves, linens, and gowns until the CDC tells them what to do with the patient's medical waste.

"We have been preparing for this since 2015," said Robin Addison, one of the nurses and co-leader of the hospital's biocontainment team. The hospital is one of about 150 nationwide ready to assess patients with Ebola or other highly contagious diseases, with training money provided by Congress.

Nationally, over 2,000 returning travelers had been screened at U.S. airports and 63 people in 22 states were being tested, although 11 of them so far have been found free of the virus, the CDC said.

China has issued massive travel bans in hard-hit sections of that country to try to stem spread of the virus. For now, CDC said it will keep up its screening process at five U.S. airports but will reevaluate its necessity.

"Right now, our response is very much a work in progress," said Sen. Richard Blumenthal, D-Conn., as lawmakers emerged from a briefing with federal health authorities. "There's a lot we're still learning."

In Chicago, the health commissioner said the newest patient hadn't had close contact with many people since becoming ill or taken public transportation or attended large gatherings.

"This is all very reassuring as far as infection risk to the general public," Arwady said.

"There is no need for the general public to change their behavior in any way."

The Great Wall of China and Shanghai Disney are among locations that are shut down amid the outbreak. Meanwhile, the capital of Beijing has canceled major events indefinitely.

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Correction, 6:57 p.m., January 24, 2020: An earlier version of this story reported that four people in New York state were under observation for possibly contracting the coronavirus. The number is actually three; the fourth person's case was proven negative, according to Gov. Andrew Cuomo's office.