By Ben Brumfield and Jacque Wilson, CNN
July 31, 2014 -- Updated 1822 GMT (0222 HKT)
NEW: CDC issues warning issues warning against "nonessential" travel to affected countries
NEW: It could take three to six months to stem the epidemic, CDC says
• Ebola hemorrhagic fever believed to have killed 729, WHO says
• Some humanitarian organizations are leaving the region to protect their own
(CNN) -- Nancy Writebol fought for her life against Ebola hemorrhagic fever on Thursday. While she did, the virus that befell the American missionary in Liberia as she worked to save its victims continued on a rampage through West Africa.
It is believed to have killed 729 people in Guinea, Liberia, Sierra Leone and Nigeria from
March through July 27, the World Health Organization said Thursday.
It is the worst Ebola outbreak in history. There is no cure and no vaccine, but care from medical workers so far has helped sustain the lives of nearly half of those stricken.
Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, said Thursday that even in a best-case scenario, it could easily take three to six months to stem the epidemic in West Africa.
The outbreak also prompted the CDC to issue a warning against all "nonessential" travel to the countries coping with an outbreak, Frieden said.
As for Writebol, her family is praying that her life is spared, too.
U.S. government officials are in ongoing talks to bring Writebol and another Ebola-stricken American, fellow aid worker Dr. Kent Brantly, back from Liberia, an administration official and a State Department source said on Thursday.
Writebol's husband, David, who is with the same mission as his wife, 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."
Writebol gets 'experimental serum'
Both Brantly, a 33-year-old who last lived in Texas, and Writebol were caring for Ebola patients in Liberia, both affiliated with faith-based international charity Samaritan's Purse.
The two were in "stable but grave condition," though Brantly took a slight turn for the worse Wednesday night into Thursday, the charity said. Writebol has been given an experimental serum, the charity said, without elaborating on what it was.
There was enough for only one person, and Brantly -- "still focused on the well-being of others," asked that Writebol get it instead of him, the charity said in a news release Thursday.
Late Wednesday, members of Writebol's church in Charlotte, North Carolina, met to pray for her struggle. Calvary Church sent her on the Liberia trip through missionary group Serving in Mission.
Liberian 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.
Record death toll
The current death toll that is the highest on record with the World Health Organization and still growing.
"This epidemic is without precedent," said Bart Janssens, director of operations for Doctors Without Borders, also known as Médecins Sans Frontières, a group of medical workers nursing victims through the disease as it runs its course. "It's absolutely not under control, and the situation keeps worsening."
Ebola fears hit close to home
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.
Presidents doubling down
The swelling cases have prompted the heads of state of two countries to cancel travel plans on Thursday to direct their full attention toward fighting the outbreak of the virus that has crippled parts of Liberia, Sierra Leone and Guinea and stirred palpable concerns that it will spread around the region and the world.
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. He announced an action plan to tear down many barriers that international medical workers say they face while fighting disease.
Ebola concern around the world
Ebola crisis is an international problem
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.
"The most challenging" aspect of trying to help people is that "we go into communities where we are not necessarily welcome," said Monia Sayah, a nurse with Doctors Without Borders.
People don't want to believe they or their loved ones have Ebola -- in part because "they understand now that the survival rate is not very high," she said. 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. On Monday, the CDC issued an alert warning travelers to avoid hospitals with Ebola patients and funerals for those patients.
The CDC also plans to send an additional 50 health specialists to help fight the outbreak, Frieden said.
On Thursday, International Air Transport Association issued a calming statement, saying it was not recommending travel restrictions to affected areas. Referring to the WHO, it said travelers there faced "extremely low" risk of contracting Ebola.
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. Further complicating matters, signs of Ebola include fever, headaches, weakness and vomiting -- symptoms that also define many other ailments, from malaria to the flu, that Brown notes often pop up "at this time of year."
Sawyer, for example, very well could have made it out of the region, perhaps to the United States, before showing symptoms of Ebola; it's only then that the virus spreads.
"If the situation does not improve fairly quickly, there is a real risk for new countries to be affected," Janssens said.
Ebola spreads through the transmission of bodily fluids. Those most at risk are loved ones of those infected, as well as health care workers tending to the ill.
Sawyer is believed to have been infected by his ailing sister, who he spent time with in Liberia, according to Brown. Neither likely knew she had Ebola.