Changes are coming for travelers who fly from West Africa to the United States. We don’t know what changes, exactly, but we know that the way passengers are screened will be altered after a man flew from Liberia to Texas and was diagnosed with Ebola.

President Obama said Monday that additional passenger screening for Ebola would take place in the United States and Africa, but he did not explain what this would entail. On Tuesday, the White House said new screening measures would be released in “the coming days.”

Thomas Frieden, director of the Centers for Disease Control and Prevention, said the agency was exploring ways to increase the screening of passengers on both sides of the Atlantic.

“We’re working very intensively on the screening process,” Frieden said during a conference call with reporters Tuesday, a week after Thomas Duncan was diagnosed in Texas. “We’re looking at that entire process to see what more can be done.”

While Frieden promised that “additional steps” were coming, he did not elaborate on what that would include. Sen. Charles E. Schumer (D-N.Y.) said in an interview Tuesday that Frieden told him the plan is to have “tougher types of screening on U.S. soil.”

However, as he has before, Frieden cautioned that any measures were meant to mitigate, rather than eliminate, the danger posed by a traveler who may have Ebola.

“We recognize that whatever we do, until the disease is controlled in Africa, we can’t get the risk to zero here,” he said “We may be able to reduce it.”

Still, Frieden said the CDC has had teams on the ground working to strengthen the screening of passengers leaving West Africa. The screening has included taking temperatures, keeping an eye out for other symptoms and having travelers fill out questionnaires.

He said the CDC methods have been used to screen more than 36,000 people over two months. And of those travelers, a small portion of whom were coming to the United States, only 77 people had a fever or other symptoms that caused them to be taken out of the line, he said. “As far as we know, none of those 77 people had Ebola,” Frieden noted.

Of course, these methods are not perfect, as was made clear when Duncan filled out a questionnaire, had his temperature taken by a person trained by the CDC and boarded multiple flights on his way to Dallas.

New measures are unlikely to fully keep out everyone with Ebola, as Frieden alluded to, owing to the realities of air travel and the disease itself. Because there are no direct flights from the three West African countries hit hardest by Ebola to the United States, a passenger could travel through a number of highly trafficked hubs as Duncan did when he went through Brussels, Dulles International Airport outside Washington and Dallas-Fort Worth. This would make a travel ban — which public health officials have repeatedly said is a bad idea — very complicated, because of the number of connections and airports involved.

In addition, because it can take up to three weeks for Ebola symptoms to emerge, a person could travel and be in another area for some time before they start feeling feverish. In this case, thermometers and a visual inspection by customs agents at U.S. airports would not be able to detect the disease.

Obama’s promise of stepped-up screening for Ebola at U.S. airports that handle international flights will fall on the shoulders of the Customs and Border Protection officers who greet passengers arriving from abroad.

Though they often seem preoccupied with scrutinizing passports and luggage, customs officers are trained to detect signs of illness, and they routinely do just that, officials said.

“CBP personnel review all travelers entering the United States for general overt signs of illnesses,” the agency said in a statement.

In addition to visual observations, CBP officers question passengers about their health and are instructed to alert the CDC if a passenger appears to be seriously ill. If a person is thought to have “a possible communicable disease,” they are isolated while the CDC and local authorities conduct the evaluation.

Federal law requires pilots to radio ahead if a passenger dies or falls ill with certain symptoms.

“If somebody comes in on a flight that has a fever or any one of those triggers, CDC can insert themselves at any point in the process,” said Rob Yingling, a spokesman for the Metropolitan Washington Airports Authority, which runs Dulles and Reagan National airports. “They can say [to the pilot], ‘Tell us more about this case,’ or they can actually physically go to the plane.”

The CDC maintains an office at Dulles and other major airports that handle international flights.

Meanwhile, up to 4,000 service members are expected to head to West African countries ravaged by the Ebola epidemic. They will be monitored several times a day, and, if any of them contract Ebola, they will be loaded onto a specially designed plane and flown back to the United States for treatment.

Among the tasks laid out for the military is the construction of 17 treatment centers for people with the disease. This will take until the middle of next month, according to Gen. David Rodriguez, chief of U.S. Africa Command.

Ashley Halsey III and Dan Lamothe contributed to this report.