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The Canadian Press, Abdeljalil Bounhar, File, Associated Press
In this Thursday, Oct. 9, 2014, file photo, a Moroccan health worker uses a thermometer to screen a passenger at the arrivals hall of the Mohammed V airport in Casablanca. Ebola has killed more than 4,500 people in West Africa and wreaked havoc on the region, but some Africans see a bright side: The virus has been limited to five countries. It has even been beaten back in two of those countries.

WASHINGTON — U.S. health officials said Thursday they still don't know how two Dallas nurses caught Ebola from a patient, as criticism increased from lawmakers who questioned whether the nation is prepared to stop the deadly virus from spreading in the country.

The revelation that one of the hospital nurses was cleared to fly on a commercial airline the day before she was diagnosed raised new alarms about the American response to the Ebola outbreak in West Africa. The death toll is expected to climb above 4,500 in Africa, all but a few within Liberia, Sierra Leone and New Guinea, the World Health Organization said.

In Sierra Leone, the government announced the virus had infected two people in the last part of the country that had been free of the disease, in the mountainous north, despite aggressive steps to keep it at bay.

In Washington, President Barack Obama directed his administration to respond in a "much more aggressive way" to the threat and, for a second day in a row, canceled his out-of-town trips to stay in town and monitor the Ebola response. He was calling foreign leaders and U.S. lawmakers to discuss what more must be done, the White House said, and bringing his Cabinet members together on the matter.

A ban on travel to the U.S. from the Ebola-stricken countries, sought by some Republican lawmakers, is not under consideration, White House spokesman Josh Earnest said Thursday.

Obama believes the U.S. already is taking the necessary steps to protect the public by screening passengers as they depart West Africa and again when they enter the U.S., Earnest said.

The first nurse stricken in the U.S., Nina Pham, who contracted Ebola after treating a Liberian man in Dallas, was being flown to the National Institutes of Health outside Washington on Thursday, while a second nurse has already been transferred to a biohazard infectious disease center at Emory University Hospital in Atlanta.

The two nurses, Pham and Amber Joy Vinson, had been involved in providing care to Thomas Duncan, who died of Ebola last week.

In a hearing on Capitol Hill, the chairman of a House committee cited "demonstrated failures" in the government's response. Rep. Tim Murphy of Pennsylvania said the "trust and credibility of the administration and government are waning as the American public loses confidence each day." Seated before him were leaders of the Centers for Disease Control and Prevention and the NIH.

Dr. Tom Frieden, director of the CDC, testified that despite the latest incidents, "we remain confident that our public health and health care systems can prevent an Ebola outbreak here."

In Europe, Spain's government is wrestling with similar questions. The condition of a nursing assistant infected with Ebola at a Madrid hospital appeared to be improving, but a person who came in contact with her before she was hospitalized developed a fever and was being tested Thursday.

That second person is not a health care worker, a Spanish Health Ministry spokesman said.

To this point, only hospital workers — the Madrid nursing assistant and the two nurses in Dallas — had been known to have contracted Ebola outside West Africa during the outbreak that began in March.

U.S. Secretary-General Ban Ki-moon said a trust fund he launched to fight Ebola, with a $1 billion goal, has a paltry $100,000 in the bank. He appealed to nations to do more about a "huge and urgent global problem that demands a huge and urgent global response." Some $20 million has been spent from the fund.

France said that on Saturday it will begin screening passengers who arrive at Paris' Charles de Gaulle airport on the once-daily flight from Guinea's capital.

In the U.S., Customs and health officials at airports in Chicago, Atlanta, suburban Washington and Newark, New Jersey, were to begin taking the temperatures of passengers from the three hardest-hit West African countries Thursday. The screenings, using no-touch thermometers, started Saturday at New York's Kennedy International Airport.

With hospitals and airports on heightened alert, Frieden said the CDC is receiving hundreds of requests for help in ruling out Ebola in travelers. So far 12 cases merited testing, he said.

Frieden said investigators are trying to figure out how the nurses caught the virus from that Liberian patient, Thomas Eric Duncan. In the meantime, he said, their cases show a need to strengthen the infection-control procedures that "allowed for exposure to the virus."

Duncan's death and the sick health care workers in the U.S. and Spain "intensify our concern about the global health threat," said Dr. Anthony Fauci, director of the Institute of Allergy and Infectious Diseases at the National Institutes of Health.

He said two Ebola vaccine candidates were undergoing a first phase of human clinical testing this fall. But he cautioned that scientists were still in the early stages of seeking new treatments or a vaccine.

A nurse at the Dallas hospital, Texas Health Presbyterian, on Thursday described a "chaotic scene" when the hospital faced Duncan, its first Ebola patient.

Briana Aguirre, who has helped treat the first nurse who was infected, told NBC's "Today" show she felt exposed in the protective gear the hospital provided.

"In the second week of an Ebola crisis at my hospital, the only gear they were offering us at that time, and up until that time, is gear that is allowing our necks to be uncovered?" Aguirre said, adding that she piled on gloves and booties in triplicate and wore a plastic suit up to her neck.

The hospital said it used the protective gear recommended by the CDC and updated the equipment as CDC guidelines changed. Because nurses complained that their necks were exposed, the hospital ordered hoods for them, according to a statement from Texas Health Presbyterian.

Frieden said nurse Amber Joy Vinson never should have been allowed to fly on a commercial jetliner because she had been exposed to the virus while caring for the first Ebola patient.

Vinson has been monitored more closely ever since Pham, the first nurse involved in Duncan's care, was diagnosed with Ebola.

Still, a CDC official cleared Vinson to board the Frontier Airlines flight from Cleveland to the Dallas area. Her reported temperature — 99.5 degrees — was below the threshold set by the agency and she had no symptoms, according to agency spokesman David Daigle.

Ebola patients are not considered contagious until they have symptoms.

Vinson was diagnosed with Ebola a day after the flight, news that sent airline stocks falling amid fears it could dissuade people from flying. Losses between 5 percent and 8 percent were recorded before shares recovered.

Frontier has taken the aircraft out of service. The plane was flown Wednesday without passengers from Cleveland to Denver, where the airline said it will undergo a fourth cleaning, including replacement of seat covers, carpeting and air filters.

Associated Press writers Emily Schmall and Nomaan Merchant in Dallas and Connie Cass and Calvin Woodward in Washington contributed to the report.