The husband of one of the patients in the U.S. with coronavirus has also been diagnosed with the illness — the first evidence that the new virus has spread person-to-person in this country, the Centers for Disease Control and Prevention reported Thursday.

"We understand that this may be concerning, but based on what we know now, our assessment remains that the immediate risk to the American public is low," Dr. Robert Redfield, director of the CDC, said during a call with reporters.

The couple lives in Chicago. The husband had not traveled to China recently, but his wife had in December to care for her father in Wuhan, which continues to be the epicenter of the outbreak.

By Thursday evening, nearly 9,000 cases had been diagnosed in China, and more than 200 people had died. The grim numbers are expected to rise again Friday.

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In the U.S., six people have now been diagnosed with the new coronavirus, two each in Illinois and California, and one each in Arizona and Washington state.

Infectious disease experts said that news of the spread in the U.S. was not unexpected.

"This is nothing to be either surprised or concerned about," said Dr. Tom Frieden, former CDC director and current president of Resolve to Save Lives, a global public health initiative. But, Frieden told NBC News, "we do need to continue to look intensively to understand both how it is spreading in China and what can be done to reduce the spread."

Both of the Chicago patients are in their 60s, and the husband has underlying medical conditions, health officials said. It's believed the husband was exposed when the wife was experiencing symptoms, which include fever and cough.

The CDC said it may take two or up to 14 days for symptoms to develop after exposure to the virus.

The woman was not symptomatic when she flew from China to her home in Illinois Jan. 13. By Jan. 24, she'd tested positive for the coronavirus and was hospitalized in isolation. Health officials said she is doing well, but remains hospitalized.

During a news conference Thursday, Dr. Allison Arwady, commissioner of the Chicago Department of Public Health, said the husband was "put into isolation when he started develop symptoms" two days ago.

That's before a test revealed he had the new coronavirus, she added. Health officials released few details about the couple, citing privacy concerns.

Doctors said the man had not been to any large public gatherings before becoming ill, and did not use public transportation in Chicago.

"We are not recommending people in the general public take precautions, such as canceling activities or avoiding going out," Dr. Ngozi Ezike, director of the Illinois Department of Public Health, said during the briefing with reporters. "Be assured that we are actively monitoring all close contacts for symptoms."

Overall, 165 samples of suspected cases have been sent to CDC for testing. Six have been positive, and 68 have been negative for the coronavirus. Test results on the rest are pending.

On Thursday, the World Health Organization declared the coronavirus outbreak a global health emergency. And late Thursday, the U.S. raised its travel advisory, telling Americans to not travel to China.

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