Story highlights Nancy Writebol is very weak but has shown signs of improvement

The missionary, who was working in Liberia, has arrived back in the United States

She is only the second known Ebola patient to be treated in the United States

A man in Saudi Arabia who was in Sierra Leone has symptoms of viral hemorrhagic fever

Nancy Writebol's family says it was making funeral plans for her last week as she lay stricken with Ebola in Liberia amid the disease's deadliest recorded outbreak

After an experimental serum and a plane flight, she's now the second human Ebola patient on U.S. soil, and her relatives think she has a fighting chance.

A medical plane on Tuesday flew Writebol from Liberia to Atlanta, where she was rushed to the same hospital where an American missionary colleague arrived days earlier. Like her, he was sickened by the deadly hemorrhagic disease while on a team caring for Ebola patients in Monrovia.

Writebol was wheeled into Emory University Hospital early Tuesday afternoon on a gurney, wearing a white, full-body protective suit and escorted by two people wearing similar gear.

There, she joins her fellow missionary Dr. Kent Brantly , who became the first Ebola patient ever in the United States on Saturday, for treatment in a special isolation unit. It is one of four of its kind in the United States designed to optimize care for those with highly infectious diseases.

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"Nancy is still very weak" but has shown signs of improvement, said Bruce Johnson, president of Christian mission group SIM USA, with which Writebol is affiliated.

Writebol's arrival contrasted with that of Brantly, who wore a similar suit but walked into the hospital Saturday with someone's assistance.

Writebol, of North Carolina, and Brantly, of Texas and Indiana, were on a joint Samaritan's Purse-SIM team caring for Ebola patients last month when they became sick in Liberia. That is one of four West African nations hit by an outbreak that the World Health Organization believes has sickened 1,603 people and killed 887 of them.

Writebol's two sons expect to communicate with her soon, Johnson said. The family was considering funeral arrangements for her just last week, days after she became sick, David Writebol said through Johnson.

"Yet we kept our faith, (and) now we have real reason to be hopeful," David Writebol said in a statement read by Johnson.

Though there is no proven treatment or vaccine for Ebola, Brantly and Writebol were recently given an experimental, U.S.-manufactured drug in Liberia while they were awaiting evacuation to the United States. Both have since shown significant improvement, sources said on condition of anonymity.

The gruesome disease that can torment victims with profuse vomiting, uncontrollable bleeding and organ failure is ravaging West Africa. The outbreak started this year in Guinea but also has affected Liberia, Sierra Leone and Nigeria.

The flight and the experimental serum

Though Writebol was weak, she had yogurt before her flight early Saturday from Liberia to the United States, Johnson said. She was taken to the plane by stretcher, but she stood up and entered the plane with assistance, he said.

The experimental drug ZMapp, which Brantly and Writebol received despite the medication never being 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 is thought to work 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.

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Photos: The Ebola epidemic Photos: The Ebola epidemic An Ebola survivor participates in a study in Monrovia, Liberia, on June 17, 2015. The country launched a five-year study to unravel the mystery of the long-term health effects that plague survivors of the viral disease. Since the epidemic started more than a year ago in a remote village in Guinea, more than 11,000 people have died, the vast majority in three West African nations, according to the latest numbers from the World Health Organization . And that number is believed to be low, since there was widespread under-reporting of cases, according to WHO. Hide Caption 1 of 45 Photos: The Ebola epidemic Women in Monrovia celebrate after the World Health Organization declared Liberia Ebola-free on May 9, 2015. Other cases have recurred since, however. Two people in Liberia have died of the disease since the end of June, just weeks after the WHO declared the nation free of the disease. Hide Caption 2 of 45 Photos: The Ebola epidemic A man walks past an Ebola awareness painting in Monrovia on March 22, 2015. Hide Caption 3 of 45 Photos: The Ebola epidemic Soldiers from the U.S. Army's 101st Airborne Division walk across the tarmac at Campbell Army Airfield before reuniting with their families at a homecoming ceremony March 21, 2015 in Fort Campbell, Kentucky. The 162 soldiers were deployed in Liberia, where they helped fight the spread of Ebola. Hide Caption 4 of 45 Photos: The Ebola epidemic Relatives weep for a loved one who it was believed died from Ebola, at a graveyard on the outskirts of Monrovia on March 11, 2015. Hide Caption 5 of 45 Photos: The Ebola epidemic Doctors Without Borders staffer Alex Eilert Paulsen watches as mattresses and bed frames burn at the Ebola Treatment Unit in Paynesville, Liberia, on January 31, 2015. The organization reduced its number of beds from 250 to 30 as gains were made in battling the virus. Hide Caption 6 of 45 Photos: The Ebola epidemic Pauline Cafferkey, a Scottish woman diagnosed with Ebola, is put on a plane in Glasgow, Scotland, on December 30, 2014. Cafferkey, a 39-year-old nurse who volunteered in Sierra Leone, was being transported to London for treatment. Hide Caption 7 of 45 Photos: The Ebola epidemic A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on November 11, 2014. Hide Caption 8 of 45 Photos: The Ebola epidemic Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on October 31, 2014. Hide Caption 9 of 45 Photos: The Ebola epidemic Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on October 30, 2014. Hickox, a nurse, recently returned to the United States from West Africa, where she treated Ebola victims. State authorities wanted her to avoid public places for 21 days -- the virus' incubation period. But Hickox, who twice tested negative for Ebola, said she would defy efforts to keep her quarantined at home. Hide Caption 10 of 45 Photos: The Ebola epidemic Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on October 28, 2014. Hide Caption 11 of 45 Photos: The Ebola epidemic U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on October 24, 2014. Pham, one of two Dallas nurses diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland. The other nurse, Amber Vinson (not pictured), was treated in Atlanta and also declared Ebola-free. Hide Caption 12 of 45 Photos: The Ebola epidemic Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on October 21, 2014. Hide Caption 13 of 45 Photos: The Ebola epidemic Health workers bury a body on the outskirts of Monrovia on October 20, 2014. Hide Caption 14 of 45 Photos: The Ebola epidemic Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on October 18, 2014, in Salisbury, North Carolina. Duncan, a 42-year-old Liberian citizen, died October 8 in a Dallas hospital. He was in the country to visit his son and his son's mother, and he was the first person in the United States to be diagnosed with Ebola. Hide Caption 15 of 45 Photos: The Ebola epidemic Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on October 15, 2014. Hide Caption 16 of 45 Photos: The Ebola epidemic Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15, 2014, in Monrovia. The group performs street dramas throughout Monrovia to educate the public on Ebola symptoms and how to handle people who are infected with the virus. Hide Caption 17 of 45 Photos: The Ebola epidemic Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12, 2014. Hide Caption 18 of 45 Photos: The Ebola epidemic A man dressed in protective clothing treats the front porch of a Dallas apartment on October 12, 2014. The apartment is home to one of the two nurses who were diagnosed with Ebola after treating Thomas Eric Duncan, a Liberian national who traveled to Dallas and later died from the virus. Hide Caption 19 of 45 Photos: The Ebola epidemic A woman crawls toward the body of her sister as a burial team takes her away for cremation October 10, 2014, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives. Hide Caption 20 of 45 Photos: The Ebola epidemic A man digs a grave on October 7, 2014, outside an Ebola treatment center near Gbarnga, Liberia. Hide Caption 21 of 45 Photos: The Ebola epidemic A person peeks out from the Dallas apartment where Thomas Eric Duncan, the first person diagnosed with the Ebola virus in the United States, was staying on October 3, 2014. Hide Caption 22 of 45 Photos: The Ebola epidemic A girl cries as community activists approach her outside her Monrovia home on October 2, 2014, a day after her mother was taken to an Ebola ward. Hide Caption 23 of 45 Photos: The Ebola epidemic A health official uses a thermometer September 29, 2014, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria. Hide Caption 24 of 45 Photos: The Ebola epidemic Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28, 2014. Hide Caption 25 of 45 Photos: The Ebola epidemic Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on September 22, 2014. Hide Caption 26 of 45 Photos: The Ebola epidemic A few people are seen in Freetown during a three-day nationwide lockdown on September 21, 2014. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes. Hide Caption 27 of 45 Photos: The Ebola epidemic Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on September 20, 2014. It was the largest single shipment of aid to the Ebola zone to date, and it was coordinated by the Clinton Global Initiative and other U.S. aid organizations. Hide Caption 28 of 45 Photos: The Ebola epidemic A child stops on a Monrovia street September 12, 2014, to look at a man who is suspected of suffering from Ebola. Hide Caption 29 of 45 Photos: The Ebola epidemic After an Ebola case was confirmed in Senegal, people load cars with household items as they prepare to cross into Guinea from the border town of Diaobe, Senegal, on September 3, 2014. Hide Caption 30 of 45 Photos: The Ebola epidemic A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on August 29, 2014. Hide Caption 31 of 45 Photos: The Ebola epidemic A burial team from the Liberian Ministry of Health unloads bodies of Ebola victims onto a funeral pyre at a crematorium in Marshall, Liberia, on August 22, 2014. Hide Caption 32 of 45 Photos: The Ebola epidemic Dr. Kent Brantly leaves Emory University Hospital on August 21, 2014, after being declared no longer infectious from the Ebola virus. Brantly was one of two American missionaries brought to Emory for treatment of the deadly virus. Hide Caption 33 of 45 Photos: The Ebola epidemic An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20, 2014. Hide Caption 34 of 45 Photos: The Ebola epidemic Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on August 19, 2014. The boy was one of the patients that was pulled out of a holding center for suspected Ebola patients after the facility was overrun and closed by a mob on August 16. A local clinic then refused to treat Saah, according to residents, because of the danger of infection. Although he was never tested for Ebola, Saah's mother and brother died in the holding center. Hide Caption 35 of 45 Photos: The Ebola epidemic Workers prepare the new Ebola treatment center on August 17, 2014. Hide Caption 36 of 45 Photos: The Ebola epidemic Liberian police depart after firing shots in the air while trying to protect an Ebola burial team in the West Point slum of Monrovia on August 16, 2014. A crowd of several hundred local residents reportedly drove away the burial team and their police escort. The mob then forced open an Ebola isolation ward and took patients out, saying the Ebola epidemic is a hoax. Hide Caption 37 of 45 Photos: The Ebola epidemic A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward August 15, 2014, in Monrovia. Hide Caption 38 of 45 Photos: The Ebola epidemic Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5, 2014. A medical plane flew Writebol from Liberia to the United States after she and her colleague Dr. Kent Brantly were infected with the Ebola virus in the West African country. Hide Caption 39 of 45 Photos: The Ebola epidemic Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20, 2014. Hide Caption 40 of 45 Photos: The Ebola epidemic Boots dry in the Ebola treatment center in Kailahun on July 20, 2014. Hide Caption 41 of 45 Photos: The Ebola epidemic Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on July 18, 2014. Hide Caption 42 of 45 Photos: The Ebola epidemic Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18, 2014. Hide Caption 43 of 45 Photos: The Ebola epidemic A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus April 3, 2014, at the European Mobile Laboratory in Gueckedou, Guinea. Hide Caption 44 of 45 Photos: The Ebola epidemic Health specialists work March 31, 2014, at an isolation ward for patients at the facility in southern Guinea. Hide Caption 45 of 45

Map: The Ebola outbreak

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 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.

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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 fatality rate of 90%, but the current outbreak in West Africa has a rate of about 60%, perhaps because of 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.

In the 1990s, an Ebola strain tied to monkeys -- Ebola-Reston -- was found in the United States, but no humans got sick from it, according to the CDC.

Concerns, testing spread outside Africa

A man hospitalized in New York City was in strict isolation Monday and Tuesday, 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. A specimen from the patient was delivered to the Centers for Disease Control and Prevention in Atlanta; testing typically is completed within 48 hours, the hospital said Tuesday.

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."

The patient was stable Monday night into Tuesday and was in "good spirits," the hospital said in a news release Tuesday.

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 was 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.