Over all, their numbers are relatively small. Of the roughly 36,000 travelers who left the three countries over the past two months, officials said, about a quarter came to the United States. Of those, 77 had symptoms, such as a fever, consistent with early-stage Ebola, but none turned out to have Ebola. Most of the fevers were related to malaria, a disease spread by mosquitoes.

The airport screenings are the federal government’s first large-scale attempt at improving the safety at American ports of entry, a measure many had called for after a Liberian man was treated for Ebola in Dallas, the first case of the disease in the United States. The man, Thomas Eric Duncan, 42, died in isolation at Texas Health Presbyterian Hospital on Wednesday.

“We work to continuously increase the safety of Americans,” said Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention in Atlanta. “We believe these new measures will further protect the health of Americans, understanding that nothing we can do will get us to absolute zero risk until we end the Ebola epidemic in West Africa.”

The C.D.C. will send personnel to airports to perform the screenings, and Coast Guard members will be deployed to help in the coming weeks.

The instrument that will be used to take people’s temperature uses an infrared beam to take the temperature. If someone has a temperature, that person will be further evaluated by a C.D.C. expert in the airports.