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A case of the new coronavirus from China has been confirmed in a patient just north of Seattle, federal health officials said Tuesday. The mysterious pneumonia-like illness has killed at least six people and sickened hundreds of others in Asia, the Centers for Disease Control and Prevention said.

In addition, the CDC said it will begin screening passengers for the virus at two more airports: Hartsfield-Jackson Atlanta and O'Hare in Chicago. The CDC started screening airplane passengers from the central Chinese city of Wuhan on Friday for signs of the respiratory illness in San Francisco, Los Angeles and New York. Now, all passengers whose flights originate in Wuhan will be rerouted to one of those five airports, it said.

The outbreak has spread from Wuhan to Beijing, Shanghai and other cities, the CDC said Tuesday. Cases have also been reported in South Korea, Thailand, Japan and elsewhere outside China.

"This is certainly not a moment for panic or high anxiety. It's a moment for vigilance."

The patient in Washington, a resident of Snohomish County, is a man in his 30s. The CDC said he arrived in the United States around Jan. 15 after having visited Wuhan. He hadn't, however, visited the seafood market where the virus is said to have originated.

Health officials said that the man had no symptoms when he arrived but that he had read about the outbreak online. When he started to develop symptoms, he contacted his health care provider.

He's in good condition but remains hospitalized "out of an abundance of caution," health officials said.

"We are grateful the patient is doing well," Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said Tuesday during a call with reporters.

In an interview later, Messonnier said "the risk to American public is low," but she added that more information is emerging day by day.

Still, "we should expect to see additional cases in the U.S. and certainly around the world," she said.

The CDC has sent a team to Washington to trace the patient's close contacts to determine whether anyone else has become ill. The man lives alone.

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Nearly all of the 300 or so cases have been reported in China, including those of at least 14 health care workers who have fallen ill with the coronavirus known as 2019-nCoV.

Washington Gov. Jay Inslee reiterated at a news conference Tuesday afternoon that the risk to state residents is low.

"This is certainly not a moment for panic or high anxiety," he said. "It's a moment for vigilance."

What is a coronavirus?

Coronaviruses are a group of viruses that can cause a range of symptoms, including runny nose, cough, sore throat and fever. Some are mild, while others are more likely to lead to pneumonia. They're usually spread through direct contact with an infected person.

There is no specific treatment for the new virus and no vaccine to prevent it. The National Institutes of Health confirmed Tuesday that it is in the "very preliminary stages" of research to develop a vaccine, but it declined to provide details.

The outbreak coincided with extensive travel in and out of China before the Lunar New Year on Jan. 25. On Wednesday, the World Health Organization will meet to discuss whether to declare the outbreak a global health emergency, which would help guide countries on how to respond, usually by offering financial or political support or both. It could also recommend against practices that could be detrimental to affected regions, such as restrictions on travel and trade.

"One thing that we've seen in outbreaks in the past is countries try to put up travel bans or propose restrictive travel in an attempt to stop the spread of an outbreak," said Alexandra Phelan, an adjunct professor at the Georgetown University law school who works on policy issues related to infectious diseases.

North Korea, for example, has reportedly closed its border to foreign tourists until the coronavirus outbreak is under control.

But, Phelan said, such policies are ineffective because people still cross borders.

"When you put travel bans in place, people don't go through the normal processes. You lose the opportunity to give people medical information, conduct appropriate screening or provide medical treatment," Phelan said.

Health officials don't know how easily the infection spreads between people. One infectious disease expert expressed concern that it could be transmitted through so-called super-spreaders — highly infectious patients able to sicken dozens of people at once.

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It's unclear whether the sickened health workers in China were all infected in the same place, but if they were, "it just smacks of a super-spreader event," said Michael Osterholm, an international infectious disease expert at the University of Minnesota.

Osterholm said super-spreader cases occurred during two well-known coronavirus outbreaks: the SARS and MERS epidemics. The 2003 SARS outbreak reached more than two dozen countries, sickening 8,098 people. Nearly 800 died.

"For those of us who dealt with SARS and MERS, it's like déjà vu all over again," Osterholm said. "When you see super-spreaders, you know you've got a problem."

There is no indication that the patient in Washington is a super-spreader.

The majority of patients have been reported in or near the city of Wuhan and have been linked to a food market with live animals. Since Chinese authorities first reported the strain late last month, the number of cases and their geographic spread have increased rapidly.

Severe cases have generally been limited to older adults with other health problems. But increasingly, Osterholm said, younger, otherwise healthy adults are falling ill. Indeed, Washington said the resident who became ill had no evidence of underlying health conditions.

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