(CNN) -- The fear began just after news broke Thursday that a long-range business jet with an isolation pod left the United States for Liberia, where it will evacuate two Americans infected with Ebola.
Twitter exploded with questions about the deadly virus, which according to the World Health Organization is believed to have killed hundreds in four West African nations. And with reaction to news that two infected Americans would soon be on their way back to the United States.
"Why are they doing this?" Robin Hunter asked in a post on Twitter.
While U.S. officials have remained mum on the issue, a source told CNN that a medical charter flight left from Cartersville, Georgia, on Thursday evening.
A healthcare worker diagnosed with Ebola after returning from Sierra Leone is wheeled in a quarantine tent onto an airplane at Glasgow International Airport in Scotland on Tuesday, December 30, bound for The Royal Free hospital in London. Health officials say the Ebola outbreak in West Africa is the deadliest ever. More than 6,000 people have died there, according to the World Health Organization.
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A CNN crew saw the plane depart shortly after 5 p.m. ET. The plane matched the description provided by the source, who spoke on condition of anonymity.
It was not immediately known when the two Americans -- identified by the source as Dr. Kent Brantly and Nancy Writebol -- would arrive in the United States, or where the plane would land.
At least one of the two will be taken to a hospital at Emory University, near the headquarters of the U.S. Centers for Disease Control and Prevention in Atlanta, hospital officials told CNN's Dr. Sanjay Gupta.
The patient will be cared for in an isolation unit at the hospital that is separate from patient areas, Gupta said.
With the return of Brantly and Writebol to the United States, it will be the first time that patients diagnosed with Ebola will be known to be in the country.
Brantly and Writebol are described as being in stable-but-grave conditions, with both reportedly taking a turn for the worse overnight, according to statements released Thursday by the faith-based charity Samaritan's Purse.
No known cure
There is no known cure or vaccine for Ebola, which the World Health Organization says is believed to have infected 1,323 people in Guinea, Liberia, Sierra Leone and Nigeria between March and July 27.
Of those suspected cases, it is believed to have been fatal in at least 729 cases, according to the health organization.
In the United States, the National Institutes of Health announced it will begin testing an experimental Ebola vaccine in people as early as September.
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The federal agency has been working on the vaccine over the last few years and says they've seen positive results when they tested it on primates.
The NIH announcement came the same day as the CDC issued a Level 3 alert for Guinea, Liberia, and Sierra Leone, warning against any nonessential travel to the region.
As of now, the outbreak has been confined to West Africa. But it could spread via travel, especially since people who have Ebola may not know it; symptoms usually manifest two to 21 days.
The symptoms include fever, headaches, weakness and vomiting, and at an advanced stage there is internal and external bleeding.
The Eboloa outbreak is believed to be the worst in history, and even in a best-case scenario, it could take three to six months to stem the epidemic in West Africa, Dr. Thomas Frieden, director of the CDC, told reporters on Thursday.
Writebol gets 'experimental serum'
Both Brantly, a 33-year-old who last lived in Texas, and Writebol were caring for Ebola patients in Liberia.
An experimental serum was administered to Writebol this week. Only one dose of the serum was available, and Brantly asked that it be given to his colleague, said Franklin Graham, president of Samaritan's Purse.
Samaritan's Purse said it did not have any additional detail about the serum.
At the same time, Brantly received a unit of blood from a 14-year-old boy who survived Ebola, the statement said. Brantly had treated the teen, it said.
It was not immediately clear what doctors hoped the blood transfusion would do for Brantly.
While blood transfusions have been tried before, Frieden told reporters no one really knows why some people survive and some don't.
There have been questions about the the health of Brantly's wife and his children, who left for Texas prior to his diagnosis.
In a statement released Thursday, Amber Brantly said she and her children "are physically fine."
"We had left Liberia prior to Kent's exposure to the virus," she said. "I am always anxiously awaiting any news from Liberia regarding Kent's condition."
Meanwhile, Writebol's husband, David, who like his wife is with Samaritan's Purse, is near her, said their son Jeremy, who spoke with CNN's Chris Cuomo from the United States.
But she is isolated from him, and he has to wear head-to-toe protective clothing similar to a hazmat suit so that he does not contract a disease that starts out with similar symptoms as a strong flu but can end in internal bleeding and death.
"Mom continues in stable condition but it's very serious, and she's still fighting," her son said. "She's weak, but she's working through it."
American doctor battling Ebola
Map: The Ebola outbreak
Map: The Ebola outbreakLiberian Information Minister Lewis Brown said his country could ill afford to lose health care workers like Writebol and Brantly.
"We join the families in prayers that they can come through this and become ... shining examples that, if care is taken, one can come out of this."
Another physician in West Africa was not so fortunate; Dr. Sheik Humarr Khan fell ill early last week while overseeing Ebola treatment at a Sierra Leone hospital and died days later.
Rate of infection
The rate of infection has slowed in Guinea, but it has increased in neighboring Sierra Leone and Liberia.
As infection accelerates, some aid groups are pulling out to protect their own.
Samaritan's Purse and the missionary group Serving in Mission have recalled all nonessential personnel from Liberia.
The Peace Corps announced Wednesday it is doing the same, removing its 340 volunteers from the three severely affected nations.
While there are no confirmed cases, a Peace Corps spokeswoman said two volunteers came into contact with someone who ended up dying from the virus.
Those Americans haven't shown signs of Ebola but are being isolated just in case. The spokeswoman said they can't return home until they get medical clearance.
Meanwhile, Liberian President Ellen Johnson Sirleaf and Sierra Leone's President Ernest Koroma both canceled trips to the United States, and Koroma declared a state of emergency. Koroma announced an action plan to tear down many barriers that international medical workers say they face while fighting disease.
Sirleaf said on CNN's "The Lead with Jake Tapper" that the country is in desperate need of people with expertise in treating and dealing with Ebola.
Some residents in affected villages have accused medical workers of bringing the disease into the country and have barricaded their towns or otherwise blocked access to Ebola victims.
Koroma said he will deploy police and military to accompany the aid workers.
They will search house to house for the infirm and enforce orders designed to curb the virus' spread.
American dies in Nigeria
One American, 40-year-old Patrick Sawyer, died in a Nigerian hospital earlier this month -- having come from Liberia. He was in a plane to Lagos, when he became violently ill. He was planning to go back home to Minnesota to celebrate his daughters' birthdays, but the disease took his life before he could.
The Nigerian government said Thursday it has located 10 more people who had contact with Sawyer, the first American who died in the Ebola outbreak. Meanwhile, none of the 67 people under surveillance and the two people in quarantine have shown symptoms of the disease, Nigerian Minister of Information Labaran Maku said.
A naturalized American citizen who worked in Liberia, Sawyer flew to Nigeria intending to attend a conference.
After exhibiting symptoms upon arrival July 20, he was hospitalized and died on July 25.
Nigeria's Minister of Health Onyebuchi Chukwu says the government is still searching for more people that had contact with Sawyer on his journey on a plane that stopped in Accra, Ghana and Lome, Togo, before traveling on to Lagos.
Ebola doctor in Sierra Leone dies
CNN's Millicent Smith, Caleb Hellerman, Jason Hanna, Jonathan Helman, Pamela Brown, Nana Karikari-apau and journalist Heather Murdock in Abuja, Nigeria, contributed to this report.
Liberian election workers watch as a voter washes her hands before casting her vote during the Senate election in Monrovia, Liberia, on Saturday, December 20. Health workers manned polling stations across Liberia on Saturday as voters cast their ballots in a twice-delayed Senate vote that has been criticized for its potential to spread the deadly Ebola disease.
U.N. Secretary-General Ban Ki-moon has his temperature measured on Friday, December 19, during a visit along with other U.N. members to an Ebola medical unit in Monrovia, Liberia. Ban is touring West Africa after criticism for a slow response to the epidemic.
Ebola health care workers carry the body of a man suspected of dying from the Ebola virus in a small village on the outskirts of Monrovia, Liberia, on Friday, December 5.
French President Francois Hollande has his temperature taken as he arrives at the Donka hospital in Conakry, Guinea, on Friday, November 28.
Liberian President Ellen Johnson Sirleaf has her temperature taken before the opening of a new Ebola clinic Tuesday, November 25, in Monrovia, Liberia.
A health worker wearing a protective suit assists patients at an Ebola treatment center in Macenta, Guinea, on Friday, November 21.
A man in New Delhi moves down the escalator at the arrivals section of the Indira Gandhi International Airport on Wednesday, November 19. A 26-year-old Indian man who recovered from Ebola in Liberia was placed in isolation at the New Delhi airport after traces of the virus were found in his semen, according to India's Health Ministry.
Health workers in protective suits transport Dr. Martin Salia from a jet that brought him from Sierra Leone to a waiting ambulance in Omaha, Nebraska, on Saturday, November 15. Salia died two days later.
A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on Tuesday, November 11.
Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on Friday, October 31.
Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on Thursday, October 30. 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.
Crew members at an airport in Accra, Ghana, unload supplies sent from China on Wednesday, October 29.
Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on Tuesday, October 28.
Amber Vinson, one of the two Dallas nurses who were diagnosed with Ebola, embraces Emory University Hospital epidemiologist Dr. Bruce Ribner after being discharged from the Atlanta hospital on October 28. Vinson and the other nurse, Nina Pham, have both been declared Ebola-free.
Hickox sent CNN this image of the tent where she was initially being isolated for Ebola monitoring Sunday, October 26, in New Jersey. Hospital officials told CNN the indoor tent was in a climate-controlled extended-care facility adjacent to a hospital.
U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on Friday, October 24. Pham, one of the two Dallas nurses who were diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland.
Police officers on Thursday, October 23, stand outside the New York City apartment of Craig Spencer, a Doctors Without Borders physician who recently returned from West Africa and tested positive for Ebola. He was later declared Ebola-free.
Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on Tuesday, October 21.
RoseAnn DeMoro, executive director of the California Nurses Association and National Nurses United, talks to reporters in Sacramento, California, after meeting with Gov. Jerry Brown to discuss the Ebola crisis on October 21.
Health workers bury a body on the outskirts of Monrovia on Monday, October 20.
Christine Wade, a registered nurse at the University of Texas Medical Branch, greets Carnival Magic passengers disembarking in Galveston, Texas, on Sunday, October 19. Nurses met passengers with Ebola virus fact sheets and were available to answer any questions. A Dallas health care worker was in voluntary isolation aboard the cruise ship because of her potential contact with the Ebola virus. She had shown no signs of the disease, however.
Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on Saturday, October 18, 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.
An airplane carrying Nina Pham arrives at an airport in Frederick, Maryland, on Thursday, October 16. Pham is one of the two nurses who were diagnosed with Ebola after treating Duncan. Pham was sent to Maryland to be treated at a National Institutes of Health hospital, and she was declared Ebola-free several days later.
Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on Wednesday, October 15.
Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15 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.
U.S. President Barack Obama speaks to the media about Ebola during a meeting in the Cabinet Room of the White House on October 15. Obama said his administration will respond to new Ebola cases "in a much more aggressive way," taking charge of the issue after the second Texas nurse was diagnosed with the disease.
A U.S. Marine looks out from an MV-22 Osprey aircraft before landing at the site of an Ebola treatment center under construction in Tubmanburg on October 15. It is the first of 17 Ebola treatment centers to be built by Liberian army soldiers and American troops as part of the U.S. response to the epidemic.
A man dressed in protective clothing treats the front porch of a Dallas apartment where one of the infected nurses resides on Sunday, October 12.
Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12.
A member of the Liberian army stands near a U.S. aircraft Saturday, October 11, in Tubmanburg.
A woman crawls toward the body of her sister as a burial team takes her away for cremation Friday, October 10, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives.
Ebola survivor Joseph Yensy prepares to be discharged from the Doctors Without Borders treatment center in Paynesville on Sunday, October 5.
Sanitized boots dry at the Doctors Without Borders treatment center in Paynesville on October 5.
Residents of an Ebola-affected township take home kits distributed by Doctors Without Borders on Saturday, October 4, in New Kru Town, Liberia. The kits, which include buckets, soap, gloves, anti-contamination gowns, plastic bags, a spray bottle and masks, are meant to give people some level of protection if a family member becomes sick.
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 Friday, October 3.
A girl cries as community activists approach her outside her Monrovia home on Thursday, October 2, a day after her mother was taken to an Ebola ward.
Marie Nyan, whose mother died of Ebola, carries her 2-year-old son, Nathaniel Edward, to an ambulance in the Liberian village of Freeman Reserve on Tuesday, September 30.
A health official uses a thermometer Monday, September 29, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria.
Children pray during Sunday service at the Bridgeway Baptist Church in Monrovia on Sunday, September 28.
Residents of the St. Paul Bridge neighborhood in Monrovia take a man suspected of having Ebola to a clinic on September 28.
Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28.
Medical staff members at the Doctors Without Borders facility in Monrovia burn clothes belonging to Ebola patients on Saturday, September 27.
A health worker in Freetown, Sierra Leone, sprays disinfectant around the area where a man sits before loading him into an ambulance on Wednesday, September 24.
Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on Monday, September 22.
A few people are seen in Freetown during a three-day nationwide lockdown on Sunday, September 21. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes.
Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on Saturday, September 20. 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.
A child stops on a Monrovia street Friday, September 12, to look at a man who is suspected of suffering from Ebola.
Health workers on Wednesday, September 10, carry the body of a woman who they suspect died from the Ebola virus in Monrovia.
A woman in Monrovia carries the belongings of her husband, who died after he was infected by the Ebola virus.
Health workers in Monrovia place a corpse into a body bag on Thursday, September 4.
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 Wednesday, September 3.
Crowds cheer and celebrate in the streets Saturday, August 30, after Liberian authorities reopened the West Point slum in Monrovia. The military had been enforcing a quarantine on West Point, fearing a spread of the Ebola virus.
A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on Friday, August 29.
Volunteers working with the bodies of Ebola victims in Kenema, Sierra Leone, sterilize their uniforms on Sunday, August 24.
A guard stands at a checkpoint Saturday, August 23, between the quarantined cities of Kenema and Kailahun in Sierra Leone.
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 Friday, August 22.
Dr. Kent Brantly leaves Emory University Hospital on Thursday, August 21, 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.
Family members of West Point district commissioner Miata Flowers flee the slum in Monrovia while being escorted by the Ebola Task Force on Wednesday, August 20.
An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20.
Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on Tuesday, August 19. 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.
A burial team wearing protective clothing retrieves the body of a 60-year-old Ebola victim from his home near Monrovia on Sunday, August 17.
lija Siafa, 6, stands in the rain with his 10-year-old sister, Josephine, while waiting outside Doctors Without Borders' Ebola treatment center in Monrovia on August 17. The newly built facility will initially have 120 beds, making it the largest-ever facility for Ebola treatment and isolation.
Brett Adamson, a staff member from Doctors Without Borders, hands out water to sick Liberians hoping to enter the new Ebola treatment center on August 17.
Workers prepare the new Ebola treatment center on August 17.
A body, reportedly a victim of Ebola, lies on a street corner in Monrovia on Saturday, August 16.
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. 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.
A crowd enters the grounds of an Ebola isolation center in the West Point slum on August 16. The mob was reportedly shouting, "No Ebola in West Point."
A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward Friday, August 15, in Monrovia.
A boy tries to prepare his father before they are taken to an Ebola isolation ward August 15 in Monrovia.
Kenyan health officials take passengers' temperature as they arrive at the Jomo Kenyatta International Airport on Thursday, August 14, in Nairobi, Kenya.
A hearse carries the coffin of Spanish priest Miguel Pajares after he died at a Madrid hospital on Tuesday, August 12. Pajares, 75, contracted Ebola while he was working as a missionary in Liberia.
Health workers in Kenema screen people for the Ebola virus on Saturday, August 9, before they enter the Kenema Government Hospital.
Paramedics in protective suits move Pajares, the infected Spanish priest, at Carlos III Hospital in Madrid on Thursday, August 7. He died five days later.
Nurses carry the body of an Ebola victim from a house outside Monrovia on Wednesday, August 6.
A Nigerian health official wears protective gear August 6 at Murtala Muhammed International Airport in Lagos.
Officials with the Centers for Disease Control and Prevention in Atlanta sit in on a conference call about Ebola with CDC team members deployed in West Africa on Tuesday, August 5.
Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5. 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.
Nigerian health officials are on hand to screen passengers at Murtala Muhammed International Airport on Monday, August 4.
Nurses wearing protective clothing are sprayed with disinfectant Friday, August 1, in Monrovia after they prepared the bodies of Ebola victims for burial.
A nurse disinfects the waiting area at the ELWA Hospital in Monrovia on Monday, July 28.
In this photo provided by Samaritan's Purse, Dr. Kent Brantly, left, treats an Ebola patient in Monrovia. On July 26, the North Carolina-based group said Brantly tested positive for the disease. Days later, Brantly arrived in Georgia to be treated at an Atlanta hospital, becoming the first Ebola patient to knowingly be treated in the United States.
A doctor puts on protective gear at the treatment center in Kailahun on Sunday, July 20.
Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20.
Boots dry in the Ebola treatment center in Kailahun on July 20.
Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on Friday, July 18.
Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18.
A doctor works in the field laboratory at the Ebola treatment center in Kailahun on Thursday, July 17.
Doctors Without Borders staff prepare to enter the isolation ward at an Ebola treatment center in Kailahun on July 17.
Dr. Mohamed Vandi of the Kenema Government Hospital trains community volunteers who will aim to educate people about Ebola in Sierra Leone.
A woman has her temperature taken at a screening checkpoint on the road out of Kenema on Wednesday, July 9.
A member of Doctors Without Borders puts on protective gear at the isolation ward of the Donka Hospital in Conakry on Saturday, June 28.
Airport employees check passengers in Conakry before they leave the country on Thursday, April 10.
A Guinea-Bissau customs official watches arrivals from Conakry on Tuesday, April 8.
Egidia Almeida, a nurse in Guinea-Bissau, scans a Guinean citizen coming from Conakry on April 8.
A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus Thursday, April 3, at the European Mobile Laboratory in Gueckedou, Guinea.
Health specialists work Monday, March 31, at an isolation ward for patients at the facility in southern Guinea.
