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The World Health Organization on Thursday declared the new coronavirus outbreak a global public health emergency.

While nearly 99 percent of the more than 9,000 confirmed cases are in China, 98 people have been diagnosed in 18 other countries, including the United States.

Late Thursday, shortly after the WHO declared the coronavirus outbreak a public health emergency of international concern, the U.S. raised its travel advisory, warning citizens to not travel to China.

.@StateDept is increasing the #China Travel Advisory to Level 4 – Do Not Travel. This is due to the spread of the novel #coronavirus throughout China & the @WHO determination that the outbreak constitutes a Public Health Emergency of International Concern. https://t.co/BIIUdavoP0 — Secretary Pompeo (@SecPompeo) January 31, 2020

The WHO's director-general, Dr. Tedros Adhanom Ghebreyesus, said the declaration is not a "vote of no confidence" in China, which has been widely praised for its transparency and work to control the outbreak.

Full coverage of the coronavirus outbreak

"The speed with which China detected the outbreak, isolated the virus, sequenced the genome and shared it with WHO and the world are very impressive," Tedros, as he's known, said Thursday during a news conference in Geneva.

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Tedros added the WHO was acting to protect countries with fewer resources.

"Our greatest concern is the potential for the virus to spread to countries with weaker health systems and which are ill-prepared to deal with it," he said.

Eight of the cases outside China were the result of human-to-human transmission; these cases took place in Germany, Japan, Vietnam and the U.S.

Because the virus is so new, much remains unknown about how it spreads or when people are contagious.

"The major gap we've seen is getting detailed epidemiology — pattern of disease spread — so we can understand where the highest risks are," Dr. Tom Frieden, former CDC director and current president of Resolve to Save Lives, a global public health initiative, told NBC News.

Many outside experts were not surprised that the WHO made the official announcement for a global emergency, which is reserved for unusual and serious public health events that have the potential to spread disease worldwide.

"This declaration may make it easier to access and mobilize further funds for resource-poor countries, and promote the need for further global cooperation," Dr. Michael Head, a senior research fellow in global health at the University of Southampton in England, said in a statement.

What's more, the WHO stressed its announcement should not disrupt any country's economy unnecessarily.

"There is no reason for measures that unnecessarily interfere with international travel and trade. WHO doesn't recommend limiting trade and movement," Tedros said.

However, the Russian government announced plans to close its border with China in an effort to keep the virus out of its country, despite the fact that infectious disease experts say viruses notoriously do not recognize such boundaries.

The emergency designation has been used just a few times in recent years, including during the 2009 H1N1 pandemic; the 2014 Ebola outbreak in West Africa; and the Zika virus epidemic in 2015-16.

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.

The Centers for Disease Control and Prevention reported the first case of person-to-person transmission in the U.S. on Thursday, between a patient and her husband in Chicago. Health officials said spread between such close contacts was not unexpected, and warned additional cases could pop up in the coming weeks.

The overall risk of spread in the U.S. remains low, officials said.

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