By Ben Brumfield and Jacque Wilson

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.

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."

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.