The World Health Organization said the fast-spreading coronavirus that's infected more than 8,200 people across the world is a global health emergency — a rare designation that helps the international agency mobilize financial and political support to contain the outbreak. The announcement comes just hours after the U.S. confirmed its first human-to-human transmission of the virus, which has killed more than 200 people in China and has now spread to at least 18 other countries. Since emerging less than a month ago in Wuhan, China, the coronavirus has infected more people than the 2003 SARS epidemic, which sickened roughly 8,100 people across the globe over nine months. As of Thursday, there are at least eight cases in four countries, outside of China, of human-to-human transmission of the new coronavirus.

'Unprecedented outbreak'

"Over the past few weeks we have witnessed the emergence of a previously unknown pathogen that has escalated into an unprecedented outbreak," WHO Director-General Tedros Adhanom Ghebreyesus said during a news conference at the organization's Geneva headquarters on Thursday. "We must act together now to limit the spread." WHO defines a global health emergency, also known as a "public health emergency of international concern," as an "extraordinary event" that is "serious, unusual or unexpected." [To see the latest updates on the coronavirus, visit CNBC's live blog here.] "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," Tedros said.

'Time for facts, not fear'

He urged the public to remain calm, saying WHO wasn't recommending "measures that unnecessarily interfere with international trade or travel." He said China was doing everything it could to contain the outbreak. "This is the time for facts, not fear. This is the time for science, not rumors. This is the time for solidarity, not stigma," he said. "To the people of China and to all of those around the world who have been affected by this outbreak, we want you to know that the world stands with you," Tedros said. "We must remember that these are people, not numbers." In a separate news conference Wednesday, Tedros said the "continued increase in cases and the evidence of human-to-human transmission outside of China are, of course, most deeply disturbing. ... Although the numbers outside China are still relatively small, they hold the potential for a much larger outbreak."

12,100 suspected cases in China

The number of confirmed cases is likely to grow, health officials said. There are more than 12,100 suspected cases in China that are either awaiting test results or haven't been tested yet, on top of the 8,137 that have already been confirmed in the country, according to WHO and China state media data released Thursday. Although the coronavirus doesn't appear to be as deadly as SARS, which had a mortality rate of about 11%, it is spreading significantly faster. The WHO data shows there are roughly 1,400 people in China who are severely ill. The illness produces a range of symptoms, with about 20% of the patients developing severe illnesses, including pneumonia and respiratory failure, he said. WHO officials said it's transmitted through human contact, in droplets through sneezing or through touching germs left on inanimate objects. WHO doesn't enact global health emergencies lightly. The international health agency has only applied the emergency designation five times since the rules were implemented in the mid-2000s. The last time WHO declared a global health emergency was in 2019 for the Ebola outbreak in eastern Congo that killed more than 2,000 people. The agency also declared global emergencies for the 2016 Zika virus, the 2009 H1N1 swine flu, and the 2014 polio and Ebola outbreaks.

Local transmission

One of the criteria used to determine whether the coronavirus is an international health threat is whether the disease spreads locally once it arrives in new parts of the world, "and that's a nuanced and important distinction to make," Dr. Mike Ryan, executive director of WHO's health emergencies program, told reporters on a call last week. The other main criteria is whether it's already interfered or will likely interfere with trade and travel, he said. The WHO committee's goal, he said, is to contain an outbreak without needlessly disrupting economic activity just by declaring a global health crisis. WHO declined at two emergency meetings last week to declare the virus a global health emergency.

Confidence in China