Larry Copeland

USA TODAY

An Ebola outbreak that has killed more than 670 people in Africa is now taking a toll on doctors and health care workers battling the deadly disease, including two Americans.

Kent Brantly, 33, an American doctor who has been working in Liberia since October for the North Carolina-based aid organization Samaritan's Purse, is receiving intensive medical treatment after he was infected with Ebola, according to a spokeswoman for the group.

Melissa Strickland said Brantly, who is married and has two children, was talking with his doctors and working on his computer while being treated.

A second U.S. citizen, Nancy Writebol, also has tested positive for Ebola, Samaritan's Purse said. Writebol is employed by mission group SIM in Liberia and was helping a joint SIM/Samaritan's Purse team treating Ebola patients in Monrovia. Writebol is married with two children, the organization said.

"Both of them tonight are in stable condition," Ken Isaacs, Samaritan Purse's vice president of programs and government relations, said Sunday. "But they are not out of the woods yet."

A Liberian government official said Sunday that one of that country's highest-profile doctors has died in what the World Health Organization (WHO) calls the largest recorded outbreak of the disease.

The Ebola epidemic in the West African nations of Liberia, Sierra Leone and Guinea has caused more than 670 deaths and more than 1,000 infections, according to the WHO. Ebola is a severe illness with a fatality rate of up to 90%, is one of the world's most virulent diseases, according to the WHO. It is transmitted by direct contact with the blood, body fluids and tissues of infected animals or people.

Over the weekend, health officials in Nigeria raced to stop the spread of Ebola after a man sick with the disease arrived on a flight in Lagos, Africa's largest city with 21 million people. He later died. The man's ability to board an international flight raised new fears that other passengers could carry the disease beyond Africa because of weak passenger inspection and the fact that Ebola's initial symptoms can resemble those of other illnesses.

Isaacs said in an interview that "where it gets really scary" is that the disease, which was previously seen only "in very remote, small villages in Africa" is now being contracted by people in the capital cities of Liberia, Guinea and Sierra Leone. "Now the disease has been introduced into the big urban areas with millions of people," he said. "In the big cities, people can get on an airplane and fly out."

Isaacs does not believe this outbreak his peaked. "I think the worst is yet to come," he said. "I hope I'm wrong."

The first Liberian doctor to die of the disease was identified as Samuel Brisbane. He was working as a consultant with the internal medicine unit at the country's largest hospital, the John F. Kennedy Memorial Medical Center in Monrovia.

Brisbane, who once was a medical adviser to former Liberian President Charles Taylor, was taken to a treatment center on the outskirts of the capital after falling ill with Ebola and died there, said Tolbert Nyenswah, an assistant health minister.

He said another doctor who had been working in Liberia's central Bong County also was being treated for Ebola at the same center where Brisbane died.

The situation "is getting more and more scary," Nyenswah said.

Isaacs said doctors and health care workers in West Africa often lack information about the disease, how it's spread and what to do if infected. Those medical professionals are often the first infected and spread the disease to their other patients. On Friday, he said, Samaritan's Purse staff saw 12 new Ebola cases; of those, eight were medical providers.

He is urging the USA, Canada and the European Union to pour resources into those countries to help them educate health care workers. "If Ebola is not fought and contained in West Africa, it will be fought somewhere else," he said.

A Ugandan doctor working in Liberia, where an Ebola outbreak has killed 129 people, died earlier this month. The current outbreak has claimed the lives of 319 in Guinea and 224 in Sierra Leone.

Last week, the medical humanitarian organization Doctors Without Borders announced that the chief doctor leading the fight against the Ebola epidemic in Sierra Leone, Sheik Umar Khan, had contracted the disease. Three nurses who worked in the same Ebola treatment Center as Khan, 39, are believed to have died from the disease.

Doctors Without Borders says it implements "strict infection control measures" to protect its staff in West Africa against the disease. "As well as the personal protective equipment that our staff wears, we have a series of strict procedures and protocols," says the group's Emergency Coordinator, Marie-Christine Ferir. "Our treatment centers are designed to ensure the safest possible working environment for our staff. There is sufficient space in between patients, clear separation between high-risk and low-risk areas, sufficient lighting, secure waste management and disinfection of the wards."

Contributing: Associated Press