Since January, officers from Customs and Border Protection have been on heightened alert for travelers who could potentially spread the virus. The Department of Homeland Security has told employees to look for visible physical symptoms and search through their travel documents and a federal database that tracks where they came from. Those customs officers will soon have to spot symptoms among a flood of more Americans funneled to designated airports from multiple countries in Europe, an administration official said, after President Trump announced new travel restrictions on the region this week.

But travelers, including some who say they showed visible signs of illness, say screening has been lax. Members of Congress this week grilled senior officials from the Department of Homeland Security over what some described as a porous screening process. And customs officers at airports question how accurately they can pinpoint people with symptoms and what safeguards are being taken to protect their health…

Maggie McDow, 46, said she had swollen glands when she landed at Dulles International Airport outside Washington, one of 11 airports where travelers who have recently been in China or Iran are being funneled. Ms. McDow had flown from London, but before that, she had stopped in South Korea, a coronavirus hot spot.

Ms. McDow said she heard plenty of instructions from airport officials for those feeling ill in airports abroad. But when she reached the United States, she said, “There was none of that.” A customs officer stamped her passport, and she was on her way.