About 60,000 to 65,000 people a year travel to the United States from Wuhan. Over the next few weeks, some 5,000 passengers are likely to be checked for signs of the new infection, Dr. Cetron said.

Only New York and San Francisco receive direct flights from Wuhan; passengers arriving in Los Angeles are on connecting flights.

Travelers will be asked to fill out questionnaires asking if they have symptoms like a cough or a fever, and whether they have visited meat and seafood markets in Wuhan, the epicenter of the outbreak.

Screeners will also use thermal scanners that can be pointed at the forehead or temple to look for fever. People with signs of illness will be examined further, along with family members or others traveling with them.

Those who seem likely to be infected with the virus will be sent to area hospitals for further testing and treatment. Dr. Cetron said hospitals in each city had been designated to handle possible cases, but declined to name them.

It may not be easy to identify likely cases at airports, because the flu season is well underway, and winter is the peak period for common colds and other respiratory viruses that can cause coughing, fever and runny noses.

The screening could take time, Dr. Cetron said, and some passengers may miss connecting flights.

The illness was first reported in late December in Wuhan, in central China. Fears of a more widespread outbreak arose when two cases were found in Thailand and one in Japan, apparently carried to those countries by air travelers from Wuhan.