BOONE, N.C. -- A second American working in West Africa has been infected with the deadly Ebola virus, an aid group said Sunday, amid what the World Health Organization is calling the largest outbreak ever recorded of the disease.

Nancy Writebol tested positive for the virus at the same medical compound in Liberia where an American doctor became infected, said Ken Isaacs, vice-president of program and government relations with U.S.-based aid group Samaritan's Purse.

Isaacs said Writebol, who works with allied aid group SIM, was in stable but serious condition at a hospital near the Liberian capital, Monrovia. The missionary was gravely ill and being treated in isolation, her husband, David, told a church elder via Skype, according to the Rev. John Munro, pastor of Calvary Church in Charlotte, North Carolina. The church has sponsored her work.

Munro said the couple, who had been in Liberia for about a year, insisted on staying there despite the Ebola threat. "These are real heroes -- people who do things quietly behind the scenes, people with a very strong vocation and very strong faith," Munro said.

Dr. Kent Brantly, the 33-year-old medical director who worked at the same Ebola care centre, was stable and in very serious condition, Isaacs said.

Brantly received intensive treatment Sunday at a hospital in Monrovia and was talking to his medical team and working on his computer, said Melissa Strickland, a spokeswoman for Samaritan's Purse, where Brantly is the medical director.

"We are hopeful, but he is certainly not out of the woods yet," Strickland said.

Early treatment improves a patient's chances of survival, and Strickland said Brantly recognized his own symptoms and began receiving care immediately.

The highly contagious Ebola virus is one of the most deadly diseases in the world. The World Health Organization says this outbreak is the largest ever recorded, killing more than 670 people in Liberia, Guinea and Sierra Leone since it began earlier this year.

There is no known cure for Ebola, which begins with symptoms including fever and sore throat and escalates to vomiting, diarrhea and internal and external bleeding.

Health workers are at serious risk of contracting the disease, which spreads through contact with bodily fluids.

Photos of Brantly working in Liberia show him in white coveralls made of a synthetic material that he wore for hours a day while treating Ebola patients.

Brantly was quoted in a posting on the organization's website earlier this year about efforts to maintain an isolation ward for patients.

"The hospital is taking great effort to be prepared," Brantly said. "In past Ebola outbreaks, many of the casualties have been healthcare workers who contracted the disease through their work caring for infected individuals."

The WHO says the disease is not contagious until a person begins to show symptoms. Brantly's wife and children had been with him in Liberia but flew home to the U.S. about a week ago, before the doctor started showing any signs of illness, Strickland said.

"They have absolutely shown no symptoms," she said.

A woman who identified herself as Brantly's mother answered a U.S. phone listing for him, but said family members were declining comment at this time.

A government official said Sunday that one of Liberia's most high-profile doctors has died of Ebola, highlighting the risks facing health workers trying to combat the deadly disease.

Dr. Samuel Brisbane was the first Liberian doctor to die in the outbreak. A Ugandan doctor working in Liberia died earlier this month.