ATLANTA » A second Dallas nurse diagnosed with Ebola was transferred Wednesday from Texas to a specialized hospital isolation unit in Atlanta that has already treated three Americans with the virus.

Helicopter footage from local television stations showed 29-year-old Amber Joy Vinson leaving a jet and being helped into an ambulance Wednesday night. A police motorcade escorted the ambulance as it traveled to Emory University Hospital.

Texas Health Presbyterian Hospital, which had been treating Vinson, confirmed her arrival in a tweet. Emory later released a statement saying "a patient with Ebola" had arrived at the hospital at 8:30 p.m.

Vinson was one of the nurses who cared for Thomas Eric Duncan, who died at the Dallas hospital last week of the Ebola virus. Another of Duncan’s nurses, Nina Pham, is also being treated for Ebola at the Texas hospital and was in "improved condition" Wednesday, said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention.

Meanwhile, an American doctor undergoing treatment for Ebola said he had been critically ill but is now recovering and expects to be discharged soon from Emory University Hospital.

The unidentified patient — a doctor working for the World Health organization at an Ebola treatment center in Sierra Leone — arrived at the hospital on Sept. 9. He said in a statement released by Emory that his condition worsened soon after he arrived but he is now much better.

The doctor is one of three American aid workers brought to Emory from West Africa; the other two recovered. Emory and three other U.S. hospitals have specialized isolation units to care for Ebola with less risk of spread to health care workers.

The WHO doctor had asked Emory to release the news about his improved condition following reports of the two recently infected Texas nurses. But he did not give his name, and hospital officials have refused to identify him, citing the wishes of the patient and his family.