The World Health Organization said the fast-spreading coronavirus that has 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 came 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.

“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.”

“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, as reported by CNBC News.