CDC Director Thomas Frieden said Sunday that Ebola screenings will likely begin by Thursday at Dulles International and three other airports, including Chicago’s O’Hare International, Atlanta’s Hartsfield-Jackson International and Newark Liberty International. (AP)

Thomas Frieden, director of the Centers for Disease Control and Prevention, said Sunday that enhanced screening of international travelers for Ebola is likely to begin Thursday at Dulles International Airport and three other gateway airports.

The announcement came with news that preliminary tests showed that a nurse at a Dallas hospital who treated an Ebola-stricken Liberian man has tested positive for the deadly virus. The worker showed symptoms after treating Thomas Duncan, a Liberian man who came down with Ebola four days after he flew to the United States on a connecting flight from Brussels.

Duncan, who died Wednesday, changed planes at Dulles and flew on to Dallas.

The enhanced airport screening, which includes taking the temperatures of passengers arriving from three African nations hit hardest by Ebola — Guinea, ­Liberia and Sierra Leone — began Saturday at New York’s John F. Kennedy International Airport.

“That screening went smoothly,” Frieden said. “There are a lot of lessons being learned through that to make sure screening goes smoothly for other passengers.”

JFK is the destination for about 43 percent of those who fly to the United States from those three countries.

The next-highest number — 22 percent — arrive at Dulles.

Speaking at a Sunday news conference, Frieden said screening would begin at Dulles, Chicago’s O’Hare International, Atlanta’s Hartsfield-Jackson International and Newark Liberty International airports by Thursday.

A CDC spokesman said it would take until then to add staff and train personnel to conduct the screening.

Martin Cetron, the CDC’s director of the Division of Global Migration and Quarantine, told reporters at JFK the screening “should add a measure of security and assurance to the American public,” but he cautioned that “no matter how many of these procedures are put into place, we can’t get the risk to zero.”

U.S. Customs and Border Protection, whose officers will provide the screening, says 94 percent of the passengers who arrive from those three nations fly into one of the five U.S. airports.

There are no direct U.S. flights from the three West African countries. Most who arrive here from those nations make connections in European hub cities.

About two dozen flights in JFK on Sunday were expected to have three or four passengers from Guinea, Liberia or Sierra Leone.

An estimated 36,000 people flew out of those three countries in the past two months, with about 9,000 of them bound for the United States, U.S. officials said. That’s an average of about 150 per day.

Any passenger who arrives at Dulles or one of the other gateway airports exhibiting symptoms that could be Ebola will be moved immediately to an isolation room staffed by CDC personnel.

Passengers from any one of the three countries who do not show symptoms will be taken to a separate inspection area.

The passengers will be instructed to complete a medical questionnaire that asks whether they have been in contact with Ebola victims. They then will be taken by a medical professional to an enclosed area where their temperature will be taken with a handle-grip, non-contact thermometer.

Countering images of workers wearing hazmat gear while treating Ebola patients, airlines and the federal agencies dealing with the virus have underscored that passengers are extremely unlikely to contract the illness simply because another passenger is showing symptoms.

The virus is not airborne and is spread through bodily fluids or infected animals.