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 arrived from Monrovia, Liberia, in a chartered plane at Dobbins Air Reserve Base and will join Dr. Kent Brantly in the isolation unit at Emory University Hospital, just downhill from the U.S. Centers for Disease Control and Prevention.

Both 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.

And while family members said both Americans have been improving after taking this novel serum, doctors at Emory have released no details about their treatment. Writebol's employer, the SIM charity, said Tuesday that she remains in serious but stable condition.

The serum was developed with U.S. military funding by a San Diego company, using antibodies harvested from mice that had been injected with parts of the Ebola virus. Tobacco plants in Kentucky are being used to reproduce it.

It's impossible to know if this treatment saved these workers from the hemorrhagic fever killing as many as 80 percent of the people the virus is infecting in Africa.

They could be recovering on their own, or for other reasons, including the better medical care that U.S. hospitals routinely provide.

Brantly, 33, and Writebol, 59, were working at a missionary clinic outside Liberia's capital. The world's largest Ebola outbreak has now spread to Guinea, Sierra Leone and Nigeria's capital of Lagos, where millions live in densely crowded conditions.

There is no vaccine or specific treatment for Ebola, but several are under development, including ZMapp, made by Mapp Biopharmaceutical Inc. of San Diego. It works by boosting the immune system's efforts to fight the virus.

The U.S. Defense Threat Reduction Agency announced on July 22 that it is providing more funding to speed the serum's development, including a critical application with the Food and Drug Administration and the production of enough of the serum to carry out human trials.

Even if that process is successful, any wider use of the serum remains many months away. But when the Americans fell sick, the charity Brantly works for, Samaritan's Purse, didn't wait.

It reached out to Emory, whose doctors contacted the drug's producers. The serum was flown to Africa, and both patients got some before the long flight to Atlanta on a plane that could only carry one of them at a time.

FDA permission must be obtained before any experimental treatments in the United States, but other countries are beyond the FDA's authority, and these aid workers were first treated in Liberia. The FDA has declined to comment on their treatment.

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. Her son, Jeremy Writebol, said he hopes her case "might help develop a cure and resources to help those who are suffering."

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 the agency's director, Dr. Tom 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 be treating them, said their families can speak with them through a plate-glass window.

But Ribner said any modern U.S. hospital could safely treat these patients using standard infection-control procedures. At least five other deadly hemorrhagic fevers have been handled at U.S. hospitals lacking such sophisticated isolation units, experts have said.

Emory's unit, one of four in the country, was built in partnership with the CDC. In 2005 it handled patients with SARS, a viral respiratory illness that spreads when carriers cough or sneeze.

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