The new virus emerged in China at the end of December and spread to other countries in Asia. The outbreak, which has sickened more than 540 people and left at least 17 dead, has triggered extensive protections around the globe, including extra screening and protocols for travelers coming from the Wuhan region in China.

In the United States, passengers from Wuhan are being funneled to airports in Los Angeles, San Francisco, Atlanta, Chicago and New York, where screening stations have been set up.

Chinese authorities escalated their own actions, announcing plans to close off the city of Wuhan by canceling planes and trains scheduled to leave the area of more than 11 million people. The World Health Organization has been considering whether to declare an international emergency.

In Washington State, officials declined to discuss additional details about the patient or the specifics of his travel, saying they would publicly disclose them only if they had trouble making contact with people who might be at risk. While public health leaders said they saw minimal risk to the public, they also said the case in Snohomish County may not end up being the only one.

“We have our first case in the U.S.,” Mr. Wiesman said. “I would expect that at some point we’re going to have more cases in the U.S.”

After the first American patient visited a clinic on Sunday, a few days after returning from China, doctors asked him to stay home and isolated as they worked with the C.D.C. to test whether he had been infected by the new coronavirus.

After confirming the illness, officials later coordinated with emergency medical technicians to bring the man to Providence Regional Medical Center in the city of Everett, where a containment unit had been built during the most recent Ebola outbreaks, said Dr. Jay Cook, the hospital’s chief medical officer. The unit has a negative airflow to filter any air coming out of the room, and an adjacent room for workers to put on protective equipment if needed.