By KATE BRUMBACK
ATLANTA (AP) - A second American aid worker infected with Ebola arrived Tuesday in Atlanta, where doctors will closely monitor the effect of an experimental drug she agreed to take even though its safety was never tested on humans.
Nancy Writebol, 59, arrived from Monrovia, Liberia, in a chartered jet at Dobbins Air Reserve Base and was then taken in an ambulance to Emory University Hospital, just downhill from the U.S. Centers for Disease Control and Prevention.
Writebol is weak but shows signs of improvement, said Bruce Johnson, president of SIM USA, the aid group with which she was working. Johnson said he had spoken with her husband, David Writebol, about her condition.
"A week ago we were thinking about making funeral arrangements for Nancy," David Writebol said in a statement read by Johnson at a news conference. "Now we have a real reason to be hopeful."
Though she was wheeled from the ambulance in a stretcher, she is progressing, according to her husband. She was able to stand up and, with assistance, get on the plane in Liberia. Before the flight, she ate some yogurt, her husband said.
Three days earlier, the other American aid worker diagnosed with the virus arrived at Emory. Dr. Kent Brantly, 33, walked from an ambulance.
The two patients - being treated in an isolation unit - were infected despite taking precautions as they treated Ebola patients in West Africa, where the virus has been spreading faster than governments can contain it, killing nearly 900 people so far.
The treatment, called ZMapp, was developed with U.S. military funding by a San Diego company, using antibodies harvested from lab animals that had been injected with parts of the Ebola virus. Tobacco plants in Kentucky are being used to make the treatment.
It's impossible to know whether the drug saved these workers, CDC Director Tom Frieden emphasized. "Every medicine has risks and benefits," he said to reporters at a health symposium in Kentucky. "Until we do a study, we don't know if it helps, if it hurts, or if it doesn't make any difference."
If the treatment works, it could create pressure to speed through testing and production to help contain the disease in Africa. Dozens of African heads of state were meeting with President Barack Obama Tuesday at a summit in Washington. But it could take years before any treatment can be proven to be effective and safe, let alone mass produced.
There is no vaccine or specific treatment for Ebola, but several are under development, including ZMapp, made by Mapp Biopharmaceutical Inc. It works by boosting the immune system's efforts to fight the virus. The U.S. Defense Threat Reduction Agency announced July 22 that it is providing more funding to speed the drug's development.
FDA permission must be obtained before any experimental treatments in the U.S., but other countries are beyond the FDA's authority. The experimental drug was flown to Africa, and these aid workers were first treated in Liberia. The FDA has declined to comment on their treatment.
Brantly and Writebol were working at a missionary clinic outside Liberia's capital. David Writebol remains in Liberia, where he said his wife's care was extraordinary.
"It's not like having a nurse come in every hour to fluff up your pillow," he said in his statement. "It's more like going into a nuclear reactor. The suits are clumsy, hot and uncomfortable. But it was like watching the love of Christ take place right before your eyes."
SIM said it's working to bring David Writebol home. The group has spent nearly $1 million since the diagnoses of Nancy Writebol Brantly, Johnson said. Samaritan's Purse, for which Brantly was working, has spent more than $1 million, Johnson said.
Ebola is spread by close contact with blood and other bodily fluids, and Writebol's duties included disinfecting doctors and nurses entering or leaving the Ebola treatment area. The virus is much less deadly when patients get top-flight care, experts say.
The CDC has been criticized for not objecting to the arrival of Ebola victims on U.S. soil, but Frieden has emphasized that there is no threat of an outbreak spreading in the United States.
Writebol and Brantly will be sealed off from anyone who isn't wearing protective gear. Dr. Bruce Ribner, who will treat them, said their families can speak with them through a plate-glass window. On Tuesday, Amber Brantly said in a statement that she has been able to see her husband every day.
Associated Press writer Bruce Schreiner in Hazard, Ky., contributed to this report.More Ebola coverage on AOL.com:
American doctors infected with Ebola given experimental drugs
Outbreak prompts airport screenings
Death toll continues to rise