The World Health Organization declared a global health emergency on Thursday as the coronavirus outbreak spread well beyond China, where it emerged last month.

The move reversed the organization’s decision just a week ago to hold off such a declaration. Since then, there have been thousands of new cases in China and clear evidence of human-to-human transmission in several other countries, including the United States.

All of which warranted a reconsideration by the W.H.O.’s emergency committee, officials said.

The declaration “is not a vote of no confidence in China,” said Tedros Adhanom Ghebreyesus, the W.H.O.’s director-general. “On the contrary, the W.H.O. continues to have confidence in China’s capacity to control the outbreak.”

The declaration comes now, he said, because of fears that the coronavirus may reach countries with weak health care systems, where it could run amok, potentially infecting millions of people and killing thousands.