In an interview, Dr. Frieden defended the existing screening process at airports in the United States, saying the costs of a more comprehensive process would not be worth the potential benefits.

“Like any intervention, there are upsides and downsides,” Dr. Frieden said. There are a lot of downsides. You slow travel. You end up costing people money.”

The best way to keep the virus out of the United States is to eradicate it at its source — in Africa, he said. But in the meantime, he noted, the risk of another infected person coming to the United States is “not zero.”

“Whatever we do, we are not going to get to zero risk in the U.S. until the outbreak is over in Africa,” Dr. Frieden said. “People may be uncomfortable with it, but we tell it like it is.”

Liberia had no screening at its airports before July 26, adding measures only after an American-Liberian man, who was visibly sick and vomited during his flight, took the disease to Nigeria, resulting in 20 infections and eight deaths.

Now, Liberia says passengers’ temperatures are taken twice by scanning their foreheads with infrared thermometers, stopping anyone with a fever over 99.5 degrees, in addition to the required health questionnaire. Officials acknowledged that the controls rely in part on the honesty of travelers, but they argued that until symptoms are evident, there were few other options.