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Pablo Martinez Monsivais, Associated Press
Centers for Disease Control and Prevention (CDC) Director Dr. Tom Frieden looks over his notes prior to testifying on Capitol Hill in Washington, Thursday, Oct. 16, 2014, before the House Energy and Commerce Committee's subcommittee on Oversight and Investigations hearing seeking answers about the Ebola outbreak from top U.S. health officials.

The director of the Centers for Disease Control and Prevention says federal health officials still do not know how two Dallas nurses caught Ebola from a patient. In Washington, lawmakers questioned whether the nation is prepared to stop the deadly virus from spreading in the U.S.

A look at the top Ebola developments worldwide:

THE LATEST

Dr. Tom Frieden testified Thursday in front of lawmakers on Capitol Hill. 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.

The chairman of a House committee said it appeared that U.S. hospitals were not ready and health care workers not properly trained or equipped.

President Barack Obama directed his administration to respond in a "much more aggressive way" to the threat and, for the second day in a row, canceled his out-of-town trips to stay in Washington and monitor the Ebola response.

THE SITUATION IN EUROPE

Spain's government is wrestling with similar issues. Three people with fever considered at risk for having Ebola were being tested Thursday in Spain for the virus, including one who arrived on an Air France jet that was isolated at Madrid's airport as a precaution.

Other passengers were allowed to disembark, but the man with fever and shivers, who had traveled from Lagos, Nigeria, was taken to a hospital.

Meanwhile, 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.

THE INFECTED NURSES

The first Dallas nurse to have contracted Ebola after treating an infected Liberian man was to be moved Thursday to a specialized medical facility in Maryland.

Nina Pham, 26, will be taken from Texas Health Presbyterian Hospital in Dallas to the National Institutes of Health in Bethesda, Maryland. The facility has one of four biocontainment units in the United States. Hospital officials said Wednesday that Pham was in good condition, and it was not immediately clear why she's being moved.

A second nurse who tested positive, 29-year-old Amber Joy Vinson, has been transferred to a biohazard infectious disease center at Emory University Hospital in Atlanta.

ERRONEOUS TWEET

University of Minnesota officials have tried to discredit a tweet claiming the school's researchers say Ebola is airborne.

University spokeswoman Caroline Marin told the Star Tribune in Minneapolis that the university never made such a claim.

In fact, the tweet refers to a commentary posted a month ago on a university website that was written by Chicago-based researchers who were debating Ebola's "potential to be transmitted" to health workers by aerosolized virus particles, and thus what protective gear they should wear.

World health authorities have been clear that Ebola is transmitted through direct contact with bodily fluids, and that blood, vomit and feces carry the most virus.

THE OVERALL TOLL

The death toll from Ebola was expected to rise this week to more than 4,500 people out of 9,000 infected, a top official with the U.N. health agency said Thursday.

Dr. Isabelle Nuttall, director of the World Health Organization's global capacities, said the focus of the world's efforts should remain on the countries where the outbreak has been spreading out of control: Liberia, Sierra Leone and Guinea.

"Our data shows that cases are doubling every four weeks," she said.