ATLANTA (CNN) -- While international health workers scramble to contain the deadliest Ebola outbreak in history, an American woman infected with the virus has arrived back in the United States.
Missionary Nancy Writebol boarded a specially equipped air ambulance Tuesday morning and headed from Liberia to Georgia. The plane made a brief refueling stop in Bangor, Maine, and was back on its way to Georgia's Dobbins Air Reserve Base by 8:45 a.m. ET, a Bangor International Airport spokeswoman said.
Once she lands in Georgia midday Tuesday, she'll be rushed to Atlanta's Emory University Hospital -- one of four hospitals in the country with extensive isolation procedures.
The hospital is just blocks away from the Centers for Disease Control and Prevention, which helped design an isolation unit at Emory to be used if a CDC staff member needed treatment for a rare deadly disease.
Writebol will be only the second person ever to be treated for Ebola in the United States. She contracted the disease in Liberia while working with SIM USA, a Christian mission group.
"We're just grateful and very cautiously optimistic about how she's doing right now," Bruce Johnson, president of SIM USA, told CNN's "Erin Burnett OutFront" Monday night.
When she arrives at Emory, Writebol will join the first Ebola patient ever to be treated in the United States.
Dr. Kent Brantly was working with Ebola patients in Liberia last month when he himself became ill. He flew to the United States on Saturday on the same specially-equipped plane and has been making progress since he arrived, a U.S. official said.
But the gruesome disease is still ravaging West Africa. More than 700 people have died in Guinea, Liberia and Sierra Leone due to Ebola, which can torment victims with profuse vomiting, uncontrollable bleeding and organ failure.
Concerns, testing spread outside Africa
A man hospitalized in New York City is now in strict isolation, waiting to learn whether he has the disease.
The patient became ill after recently traveling to West Africa, New York's Mount Sinai Hospital said.
Doctors were trying to confirm the cause of the man's high fever and gastrointestinal symptoms Monday. Results from an Ebola test are expected Tuesday or Wednesday.
But "odds are this is not Ebola," said Dr. Jeremy Boal, chief medical officer of the Mount Sinai Health System. "It's much more likely that it's a much more common condition."
CNN Chief Medical Correspondent Dr. Sanjay Gupta agrees. About half a dozen people have recently returned from West Africa and gotten tested because of symptoms, but none of those cases has been confirmed as Ebola, Gupta said.
Doctors in Saudi Arabia are also taking precautions as they treat a 40-year-old man who recently returned from Sierra Leone.
The man is in critical condition Tuesday with symptoms of a viral hemorrhagic fever, the Saudi Health Ministry said.
The source of his infection remains unknown, but Ebola cannot be ruled out, the ministry said.
How Ebola spreads
Ebola doesn't spread through the air or water. The disease spreads through contact with infected organs and bodily fluids such as blood, saliva and urine.
Historically, the odds have not been good. Previous Ebola outbreaks have had a morbidity rate of 90%, but the current outbreak in West Africa has a fatality rate of about 60% due to early treatment.
There is no FDA-approved treatment for Ebola. Emory will use "supportive care" for its two Ebola patients, unit supervisor Dr. Bruce Ribner said.
That means carefully tracking a patient's symptoms, vital signs and organ function and using blood transfusions and dialysis to keep patients stable.
The National Institutes of Health plans to begin testing an experimental Ebola vaccine in people as early as September. Tests on primates have been successful.
The secret serum
But the experimental drug ZMapp, which has never been subjected to clinical trials, is getting a lot of attention.
Just last Thursday, Brantly's condition in Liberia had deteriorated so badly that he called his wife to say goodbye.
But three vials of ZMapp stored at subzero temperatures were flown into Liberia. Brantly and Writebol took the drug, and their conditions improved before they evacuated to the United States.
The medicine works by preventing the virus from entering and infecting new cells. It's a three-mouse monoclonal antibody -- meaning mice were exposed to fragments of the Ebola virus, and the antibodies generated within the mice's blood were harvested to create the medicine.
While Brantly and Writebol's conditions improved after taking the drug, the serum shouldn't be viewed as a miracle cure, internist and gastroenterologist Dr. Jorge Rodriguez said.
"Let's be cautious. We don't even know really if this serum is working," Rodriguez said. "I'm glad now that these patients were brought to a hospital where so many tests can be done, where they can see the response of their body to this serum. We don't know if these patients are naturally getting better, or whether the serum is really doing something."
Many have asked why the two Americans received the experimental drug when so many -- about 1,600 people -- in West Africa also have the virus.
The World Health Organization says it was not involved in the decision to treat Brantly and Writebol. Both patients had to give consent to receive the drug, knowing it had never been tested in humans before.
The process by which the medication was made available to the American patients may have fallen under the U.S. Food and Drug Administration's "compassionate use" regulation, which allows access to investigational drugs outside clinical trials.
CNN's Danielle Dellorto, Jacque Wilson and Caleb Hellerman contributed to this report.