When they landed, U.S. Customs and Border Protection agents wearing paper and plastic masks boarded the plane and handed passengers a questionnaire to fill out before leaving the plane. On the jetway, an agent took each person’s temperature.

John Mixon, 86, a retired law professor, said he waited nearly two hours at Kennedy International Airport after arriving from Amman, Jordan, missing his connecting flight to Houston because of the slow screening process.

Mr. Mixon said he and his wife were shuffled into a packed room with more than 100 others for hours before a few C.D.C. officials came in and asked everyone if they had been to China or Italy. Only one man raised his hand, and everyone else was told they could leave after getting their temperatures taken, Mr. Mixon said.

Since January, officers from Customs and Border Protection have been on heightened alert for travelers who might spread the virus. The Department of Homeland Security has told employees to look for physical symptoms, search through travel documents and review a federal tracking database.