The World Health Organization (WHO) has declared the outbreak of the new coronavirus spreading quickly across China a global health emergency — a rare designation the agency gives disease threats that pose an international risk.

The decision Thursday came as the number of people diagnosed with the 2019-nCoV virus has skyrocketed to more than 8,200, surpassing the SARS case toll. There are also now people in at least 18 other countries with the virus.

“Over the past few weeks we have witnessed the emergence of a previously unknown pathogen which has escalated into an unprecedented outbreak,” WHO director general Tedros Adhanom Ghebreyesus said.

Just a week ago, the agency determined the formal “public health emergency of international concern,” or PHEIC, declaration, was premature.

.@WHO is working closely with all countries to control the new #coronavirus outbreak. Person-to-person transmission has been seen in at least 3 countries outside #China – Japan , Vietnam & Germany - and the cooperation and info-sharing has been very good. — Tedros Adhanom Ghebreyesus (@DrTedros) January 30, 2020

But since then, the situation has changed dramatically. According to the WHO, there are now 98 cases outside of China in 18 countries, including eight instances of human-to-human transmission in four countries. The vast majority of these have been linked to China’s Hubei province, where the outbreak originated.

“Although these numbers are still relatively small ... we must all act together now to limit further spread,” Dr. Tedros said, framing the declaration as a precautionary measure. He also flagged the WHO’s concern with the potential damage the virus could do in countries with weaker health systems, calling on the international community to help.

This is the sixth time the WHO has declared a public health emergency

A PHEIC — pronounced “fake” — is defined as “an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response.”

In reality, it’s a political tool the WHO can use to draw attention to a serious disease that has caught the world off guard and put people’s health in danger. It’s meant to engage the global community in a coordinated outbreak response, galvanize resources, and stop the disease from spreading further across borders.

Related The 2 key questions that will determine if the coronavirus outbreak becomes a pandemic

Before Thursday, the WHO had only declared a public health emergency five times since the 2007 enactment of the International Health Regulations, which govern global health emergency responses.

Several global health experts have been calling since last week for the agency to reconsider its decision about the novel coronavirus. “It’s long overdue, and it should have happened Monday,” Devi Sridhar, professor and chair of Global Public Health at the University of Edinburgh, told Vox.

“This is an international emergency,” said Tom Frieden, the former director of the US Centers for Disease Control and Prevention. “A PHEIC allows [the WHO] to further lean into the role of global leadership for governments and the private sector.”

The outbreak was first reported to the WHO by Chinese officials a month ago, on December 31. The leading hypothesis was that the virus emerged in Wuhan, a city of 11 million in Hubei province, spreading directly from animals to humans in a market there.

Scientists are still trying to suss out the origins of the outbreak, but the virus has since spread beyond Wuhan — and to 18 other countries. Sridhar pointed out that one of the reasons the WHO pauses before triggering a PHEIC is because of the economic fallout international attention on a disease threat can bring. According to Reuters, the global economic losses from the 2003 SARS outbreak totaled $40 billion, and the world’s gross domestic product “suffered a 0.1% hit due to the outbreak.”

But in the past week, countries have already sealed their borders to China, issued travel advisories, and started repatriating citizens. Australia took the extraordinary measure of sending returning citizens from Wuhan, the center of China’s outbreak, to a remote island 1,200 miles off the coast of the mainland. Meanwhile, numerous airlines — including British Airways — announced they would suspend travel to the country.

“Right now we need some guidance,” Sridhar added. “Everyone has been waiting for the WHO. The world is moving on with or without the WHO.”

In a press briefing Thursday, Dr. Tedros said the international community should help countries with weaker health systems prepare for outbreaks, that countries should work to combat the spread of false information and rumors, and share data and knowledge about the virus, among other recommendations.

“This is the time for science, not rumors,” Dr Tedros said. “This is the time for solidarity, not stigma.”

The agency didn’t call for any travel or trade restrictions, such as border closures, visa refusals, or quarantining people who are in good condition.

Dr. Tedros also applauded China’s efforts. “We would have seen many more cases outside of China by now,” he said. “In many ways, China is setting a new standard for outbreak response.”

The outbreak is still centered on mainland China

Just because the emergence of the new coronavirus is now officially a global health emergency doesn’t mean it’ll become a pandemic. It’s important to note the outbreak is still heavily centered on mainland China. Of the 8,235 confirmed cases, 8,124 have been found in mainland China, with more than half of those in Hubei province.

There’s also still much that’s unclear about 2019-nCoV, including how lethal it is. In humans, coronaviruses can lead to symptoms that range from the common cold to severe pneumonia and death in the case of SARS and MERS. It’s not yet known where 2019-nCoV falls on that spectrum, and it’ll be weeks before researchers have a clearer picture. But we do know that in addition to the 171 deaths the virus has caused, there have been many reports of people with very mild symptoms. There’s also evidence of asymptomatic cases.

It’s possible that as we learn more, 2019-nCoV will look more like the common cold than like SARS. That’s because infectious diseases typically look more severe when they’re first discovered, since the people showing up in hospitals tend to be the sickest. Once more of these mild or asymptomatic cases are discovered, this virus could wind up looking a lot less scary. (For more on that, see this Vox explainer.)

“We live [with] and tolerate a whole lot of respiratory viruses,” said Jennifer Nuzzo, senior scholar at the Johns Hopkins Center for Health Security, “some of which are even more transmissible than the estimates people have come out with for this one — but they don’t make the headlines.” If the new coronavirus winds up looking less severe, she added, “We may be moving away from containing the virus as a goal to one of minimizing its spread.”