A second American infected with Ebola in Liberia was showing "continued improvement" as she arrived in the U.S. for treatment in an Atlanta hospital.

Nancy Writebol, 59, an aid worker, was flown Tuesday to Dobbins Air Reserve Base in Marietta, Georgia, then taken to Emory University Hospital. Kent Brantly, 33, a doctor who contracted the virus at the same treatment center in Liberia as Writebol, has been receiving medical care at Emory since Saturday.

While Writebol was "very, very weak," she was showing signs of "continued improvement," said Bruce Johnson, the president of SIM USA, the Charlotte, North Carolina-based charity that sponsored her work in Liberia.