The World Health Organization on Thursday declared a global health emergency in the face of increasing anxiety over a virus in China that has caused 170 deaths and infected more than 7,834 people.

The decision came shortly after the Centers for Disease Control and Prevention announced that the United States had its first case of "person-to-person" transmission of the disease, known as novel coronavirus. Previous cases in the country had involved someone who had traveled to Wuhan, China, believed to be the source of the outbreak. But the latest individual to test positive for the virus had no history of travel to Wuhan. The man, who lives in Illinois and is said to be in his 60s, is married to a woman who had recently returned from Wuhan, according to the Illinois Department of Public Health.

Across the globe, there have been eight cases of person-to-person transmission.

On Thursday, a spokesperson for the New York City Health Department said that there had not yet been any suspected cases in the city. The state said there were now 11 people who have been sent for testing, one more since Tuesday, but no confirmed cases. The tests are being performed by the CDC, and state health officials said they were currently awaiting the results for four people.

Despite deciding to upgrade the status of the epidemic, WHO officials cautioned the public against panicking about the disease. The virus has been relatively contained; all of the deaths and 99 percent of infections—7,736 cases—have occurred in China.

Tedros Adhanom Ghebreyesus, the WHO director-general, said the reason for changing its stance was to protect countries that may not be able to handle an outbreak. All least 18 countries now have reported infections.

"We don’t know the sort of damage this virus would do to a country with a weaker health system," he said.

"Let me be clear," he added. "This declaration is not a vote of no-confidence in China. On the contrary, the WHO continues to have confidence in China’s ability to control the outbreak."

The designation allows the WHO to issue global guidelines to handle the outbreak and also encourage cooperation and sharing of resources among affected countries.

"It means they should come together and deal with this epidemic as one unit," said Dr. Nischay Mishra, a virologist and molecular biologist at Columbia University's Mailman School of Public Health. "That's the whole plan."

He and his colleagues have been racing to develop a diagnostic test that can detect the virus in people who do not show any symptoms. The lab where he works is led by Dr. W. Ian Lipkin, who advised the Chinese government and WHO during the SARS epidemic and who is currently in China.

In New York City, which is said to have the largest Chinese community outside of Asia, pharmacies have been running out of face masks amid Coronavirus fears, particular among Chinese New Yorkers. The possibility of a shortage has worried public health officials that doctors and nurses who need the masks will be unable to get them.

Health experts have downplayed the efficacy of face masks in preventing infection, especially in outdoor settings. Federal and local health officials this week said that in light of the low risk of danger, face masks are unnecessary. They are, however, advising the people take other standard public health precautions like washing their hands regularly and covering their mouths with a tissue or elbow when coughing or sneezing.

Dr. Perry Pong, the chief medical officer of the Charles B. Wang Community Health Center in Chinatown, said his staff had begun limiting the number of masks they hand out because patients had been taking more than they needed. He acknowledged that some people may simply take comfort from wearing a mask.

"It may make them feel more secure, but it’s not recommended as a general measure," he said.

He said he was worried about panic in the community, particularly on social media. Earlier this week, there was a report circulating on WeChat, a Chinese messaging app, that a patient suspected of having coronavirus had checked into the Charles B. Wang Community Health Center and was transported to Mount Sinai Beth Israel hospital.

"I can unequivocally say there has been no truth to that rumor," he said.

Dr. Mishra said that the fact that New York City had not yet seen the virus suggests that New Yorkers may have canceled their travel plans to China in light of the disease, which was first reported in late December. He said it also showed good screening and collaboration between state and local officials and public health experts.

"If there is a case in New York tomorrow, we will work with both agencies," he said. "So far everything is under control."