The head of the World Health Organization said Wednesday its emergency committee needed more information before it was ready to declare the deadly global coronavirus virus outbreak a public health emergency, even as the hard-hit Chinese city of Wuhan shut down all air and train traffic to try to contain its spread.

The death toll from a new global virus rose to at least 17 on Wednesday while experts from around the world were meeting in Geneva to debate how to address the ominous outbreak.

WHO director-general Tedros Adhanom Ghebreyesus called the issue an "evolving and complex situation" that required further study.

While praising China's cooperation in presenting information about the virus, the WHO leader called for more debate on Thursday.

"The decision about whether or not to declare a public health emergency of international concern is one I take extremely seriously, and one I am only prepared to make with appropriate consideration of all the evidence," he said.

China has identified more than 500 cases and all the deaths, mostly in and around the central city of Wuhan. The virus has spread to other parts of China, and cases have also been confirmed in the U.S., Thailand, South Korea, Japan and Taiwan.

The first U.S. case, reported Tuesday, involves a Washington state resident in his 30s who first displayed symptoms Sunday. He was listed in good condition at Providence Regional Medical Center in Snohomish County north of Seattle.

"Our first priority remains public safety,” said Washington state Secretary of Health John Wiesman. “We believe the risk to the public is low. And as this situation evolves, we will continue to communicate with the CDC, Snohomish County and the public.”

Coronaviruses are a large family of viruses that cause illness ranging from the common cold to more severe infections such as pneumonia, Middle East respiratory syndrome (known as MERS) and severe acute respiratory syndrome (SARS). Common signs of infection include fever, cough, shortness of breath and breathing difficulties. In more severe cases, infection can cause pneumonia, kidney failure or death.

A declaration by WHO of a public health emergency would be a prerequisite for issuing recommendations to countries on guidelines to combat it.

To reduce the spread of the virus, the Wuhan local government canceled all flights and trains from the city for Thursday and temporarily closed all public transportation – bus, subway, ferry and long-distance bus service.

The official Xinhua News Agency said Thursday that the city asked people not to leave Wuhan without specific reasons. Chinese Vice Premier Sun Chunlan ordered an all-out effort to curb the virus outbreak.

Trish Perl, a coronavirus expert at UT Southwestern Medical Center in Dallas, said health officials should take seriously the threat of the virus spreading across the U.S.

"We don't know the magnitude of this outbreak, but we know from previous outbreaks that coronaviruses can spread quickly," Perl told USA TODAY. "Some people may not know they're infected."

The U.S. representative at the WHO meeting is Martin Cetron, who heads the division of Global Migration and Quarantine at the Centers for Disease Control and Prevention. WHO has been here before, recommending emergency declarations made for outbreaks of SARS, MERS, Ebola, Zika and swine flu.

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In China, the timing of the outbreak could not be worse – the Lunar New Year is Saturday, and hundreds of millions of people across Asia travel in packed buses, trains and planes to celebrate. Even before the halting of transportation, Chinese health officials had urged the 11 million residents of Wuhan not to leave the city, to avoid crowds as much as possible and to wear masks in public places.

To scan travelers, the city has installed 35 infrared thermometers and more than 300 portable infrared thermometers in the city's airports, railway and bus stations since Jan. 14, according to the Health Commission of Wuhan, the China Daily reported.

All passengers with higher than normal temperatures will be registered and transferred to nearby medical facilities, the commission said.

In addition, pharmacies limited sales of face masks to one package per customer as people lined up to buy them. Residents said they were not overly concerned as long as they took preventive measures.

A SARS outbreak that spread from China across several countries in 2002-2003 killed 800 people and sickened more than 8,000.

"The fact that the coronavirus is in the same family as MERS and SARS, and that we do not know how contagious it currently is, is worrying," said Ogbonnaya Omenka, an assistant professor and public health specialist at Butler University's College of Pharmacy and Health Sciences. "The U.S. should be hypervigilant, especially now that there has been a reported case in the country."

Coronaviruses are zoonotic, meaning they are transmitted between animals and people. This illness is now also being spread between people, health officials said. The source of the outbreak is still under investigation in Wuhan. Preliminary investigations have identified the now-closed Huanan Seafood Wholesale Market.

The CDC, which began enhanced health screenings last week at airports in San Francisco, New York and Los Angeles, said Atlanta and Chicago would be added to the list. All traveling from Wuhan to the U.S. are rerouted to these airports.

New Jersey's senators, Democrats Robert Menendez and Cory Booker, asked the CDC to expand the list to include all major international U.S. airports, specifically Newark Liberty International Airport.

President Donald Trump told reporters Wednesday that his administration has a plan to block any outbreak of the coronavirus, saying the CDC "has been terrific."

There is no vaccine, although nine studies are examining coronavirus vaccine development. There is also no specific treatment, but recommended measures are similar to those for a cold, such as rest and drinking a lot of fluid.

WHO said basic measures to curb spreading the virus include avoiding close contact with people suffering from acute respiratory infections, frequent hand-washing and avoiding unprotected contact with farm or wild animals.

WHO does not recommend any specific health measures for travelers. In case of symptoms that suggest acute respiratory illness during or after travel, travelers are encouraged to seek medical attention and share their travel history with their health care provider.

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