(CNN) -- A second American suffering from Ebola is expected to arrive in Atlanta on Tuesday from Liberia, where she contracted the deadly virus.
Missionary Nancy Writebol will travel aboard an air ambulance equipped with an isolation unit. It will land at Dobbins Air Reserve Base, and from there she'll be rushed to Atlanta's Emory University Hospital about 20 miles away.
Writebol will be the last of two Americans stricken with the disease while aiding Ebola victims in West Africa. Ebola has killed more than 700 people in three nations: Guinea, Liberia and Sierra Leone.
Their evacuation to Atlanta marks the first time anyone infected with the virus has been known to get treatment in the United States.
Both patients will be treated at an isolation unit where precautions are in place to prevent it from spreading, unit supervisor Dr. Bruce Ribner said.
The first evacuee -- American Dr. Kent Brantly -- was making progress since he arrived in Atlanta from Liberia on Saturday, a U.S. official said.
"It's encouraging that he seems to be improving," Thomas Frieden, director of the Centers for Disease Control and Prevention, told CBS' "Face the Nation."
"That is really important, and we are hoping he will continue to improve."
Brantly, 33, is the first known patient with the deadly virus to be treated on U.S. soil. He landed at Dobbins Air Reserve Base in Georgia and was quickly rushed toAtlanta's Emory University Hospital.
Phoenix Air says its highly specialized air ambulance, equipped with an isolation unit, departed Georgia for Liberia on Sunday evening to pick up Writebol. The flight is scheduled to land in Georgia on Tuesday.
Both Brantly and Writebol became sick while caring for Ebola patients in Liberia, one of three West African nations hit by an outbreak.
This will be the first human Ebola test for a U.S. medical facility. The patients will be treated at an isolated unit where precautions are in place to keep such deadly diseases from spreading, unit supervisor Dr. Bruce Ribner said.
Ebola doesn't spread through airborne or waterborne methods. It spreads through contact with organs and bodily fluids such as blood, saliva, urine and other secretions of infected people.