A man in his 30s in Washington State is infected with the Wuhan coronavirus, the first confirmed case in the United States of a mysterious respiratory infection that has killed at least six people and sickened hundreds more in Asia, the Centers for Disease Control and Prevention announced on Tuesday.

Federal officials also announced expanded screenings for the infection at major airports in the United States. In addition to New York, Los Angeles and San Francisco, airports in Atlanta and Chicago will begin examining passengers arriving from Wuhan, China, for signs of illness.

The infected man, a resident of Snohomish County, Wash., developed symptoms after returning from a trip to the region around Wuhan where the outbreak began.

He returned from Wuhan on Jan. 15, two days before passenger screening was instituted at three major airports in the United States, but he had no symptoms at the time.

He read online about the mysterious new virus that causes fever and respiratory illness, and informed his doctors about his recent trip when he sought care on Sunday, federal officials said.

He was seen at a clinic in Snohomish County and is now hospitalized at Providence Regional Medical Center-Everett. Specimens from the patient were sent to the C.D.C. in Atlanta, and officials confirmed on Monday that he was infected with the Wuhan coronavirus, also called 2019-nCoV.

Health officials declined to identify the patient, but said that he has a mild case of pneumonia, is doing well and has been very cooperative.

Health care workers are also trying to identify people who may have had contact with the infected traveler, and are notifying some of the passengers on his return flight who might have been exposed. They will monitor those fellow passengers for fever and respiratory symptoms.

“We’re very comfortable the patient is isolated and poses little risk to the staff or general public,” said Dr. Chris Spitters, health officer at the Snohomish Health District, adding that the strict isolation practices were implemented in “an abundance of caution.”

News of the first case of the virus in the United States surfaced as officials studied growing evidence that the disease can be transmitted from person to person, although it is not clear how easily.

“There is new information hour by hour, day by day, that we are tracking and following closely,” said Dr. Nancy Messonnier, director of the C.D.C.’s National Center for Immunization and Respiratory Diseases.

The C.D.C. has deployed staff overseas who are working closely with the ministries of health in China and Thailand. “The key issue we all need to understand is how easily and sustainably the virus is spread from human to human,” Dr. Messonnier said.

Federal health officials plan to expand the screening of passengers arriving in the United States from Wuhan to five airports that are major international hubs. All travelers from the city will be funneled through those airports.

Screenings started on Friday and Saturday at New York’s Kennedy International Airport, San Francisco International and Los Angeles International. Now they will be expanded to Hartsfield-Jackson Atlanta International and Chicago O’Hare International over the next few days.

Passengers from Wuhan planning to arrive in other cities will be issued new tickets taking them to one of the five screening airports, officials said.