WASHINGTON — The revelation that a second Dallas nurse who is ill with Ebola was cleared to fly the day before her diagnosis raised new alarms as leaders of the nation’s public health system prepared to defend their efforts to contain the deadly virus before a congressional hearing Thursday.

President Barack Obama directed his administration to respond in a “much more aggressive way” to oversee the Dallas cases and ensure the lessons learned there are transmitted to hospitals and clinics across the country. For the second day in a row he canceled out-of-town trips Thursday to stay in Washington and monitor the Ebola response.

Federal health officials who say they know how to shut down the disease within the U.S. were being called to testify in what was looming as a combative hearing by a House oversight panel on Capitol Hill.

In prepared testimony, Dr. Anthony Fauci, director of the Institute of Allergy and Infectious Diseases of NIH, said that the death of a Liberian man in Dallas and the subsequent infections of the two Dallas nurses as well as anEbola diagnosis of a nursing assistant in Spain “intensify our concerns about this global health threat.” He said twoEbola 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 understanding how Ebola infection can be treated and prevented.

Spain’s government is wrestling with similar questions. The condition of the nursing assistant, who was infected 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 for the virus Thursday.

That 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 — are known to have contracted Ebola outside West Africa in this outbreak, which is spreading out of control in Liberia, Sierra Leone and Guinea.

A nurse at the Dallas hospital, Texas Health Presbyterian, on Thursday described a “chaotic scene” when the hospital faced its first Ebola patient, Liberian traveler Thomas Eric Duncan.

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 U.S. Centers for Disease Control and Prevention, 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.

CDC Director Tom Frieden said that 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 Ebola patient who traveled from Liberia.

Vinson was being monitored more closely since another nurse, Nina Pham, also 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 in afternoon trading.

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.

Amid increasing anxiety over Ebola, Customs and health officials at airports in Chicago, Atlanta, the Washington suburbs and Newark, New Jersey, were scheduled Thursday to start 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.