As the health of two Ebola-stricken American missionaries deteriorated late last month, an international relief organization backing them hunted for a medical miracle. The clock was ticking, and a sobering fact remained: Most people battling the disease do not survive.

Leaders at Samaritan’s Purse, a North Carolina-based Christian humanitarian group, asked officials at the Centers for Disease Control and Prevention whether any treatment existed — tested or untested — that might help save the lives of Kent Brantly and Nancy Writebol, both of whom had contracted Ebola while helping patients in Liberia.

The CDC put the group in touch with National Institutes of Health workers in West Africa, where an employee knew about promising research the U.S. government had funded on a serum that had been tested only in monkeys.

“Our staff in Liberia knew about the research and flagged it for the religious groups,” said Anthony Fauci, director of the NIH’s National Institute of Allergy and Infectious Diseases.

Within days, doses of the unproven treatment had made their way in frozen vials across the ocean and were administered to Brantly and Writebol.

“This so-called experimental serum is a cocktail of antibodies that have the capability of blocking the virus,” Fauci said, adding: “The physicians in charge of the patients’ care made a risk-benefit decision. The risk was less than the potential benefit.”

While it is too early to say whether the treatment saved the lives of the two missionaries or slowed the disease’s progression enough to allow them to return to the United States for care, some reports have suggested that Brantly and Writebol improved after getting the serum.

Both Samaritan’s Purse and CDC Director Tom Frieden have described Brantly’s condition as “improving.”

Palmer Holt, a spokesman for SIM, or Service in Mission, a Christian relief group that employs Writebol and her husband, said Writebol has had good days and bad days. But he added that while her condition was worsening on July 30, the day before she first received the treatment, she seemed to stabilize in the days that followed.

Holt said that Writebol, who is scheduled to arrive at Emory University Hospital in Atlanta on Tuesday, has received a second dose of the serum, with encouraging results. “She’s showing improvement,” he said in an e-mail Monday. “She is walking with assistance . . . strength is better . . . has an appetite.”

There is no approved cure for Ebola and no proven vaccine to prevent the disease. Part of the problem, beyond the difficulty in developing reliable treatments, has been a lack of interest from drug companies, given that Ebola has affected relatively few people compared with many other diseases.

The drug cocktail the two Americans received, known as ZMapp, was developed by the San Diego company Mapp Biopharmaceutical. It is manufactured in Kentucky using fast-growing tobacco plants, which act as “photocopiers” to produce proteins that are extracted from the plants and processed into the drug, said a spokeswoman for Kentucky BioProcessing, the company that works with Mapp. CNN first reported the name of the serum given to the Americans.

As the worst Ebola outbreak in history unfolds in West Africa, The Post's Joel Achenbach explains how the deadly virus wreaks havoc on the human body. (Davin Coburn/The Washington Post)

The medication builds on previous efforts to develop anti-Ebola drugs — one that involved the U.S. Army Medical Research Institute of Infectious Diseases and another that was a collaboration between the Public Health Agency of Canada and a private company in that country.

According to a 2012 study in the journal Science Translational Medicine, all four monkeys survived after receiving a precursor of ZMapp a day after being infected with the Zaire strain of the virus, the one that is currently infecting West Africans. In another group, two out of four monkeys survived when given the drug 48 hours after infection.

ZMapp was first identified as a drug candidate in January, and very little of it is available, according to Mapp. The company said it and its partners “are cooperating with appropriate government agencies to increase production as quickly as possible.”

Last week, federal officials announced that they have fast-tracked development of an Ebola vaccine and plan to start human trials next month. The potential vaccine has shown encouraging results in primates. An early trial primarily would determine whether the vaccine is safe and would involve about 20 people. If all goes well, the vaccine could be available on a limited basis sometime in 2015.

It is unclear whether Brantly and Writebol will continue to receive doses of the ZMapp serum during their treatment in Atlanta. Any use of an experimental drug in the United States must receive the blessing of the Food and Drug Administration.

Citing federal privacy laws, an FDA spokeswoman on Monday declined to say whether the agency had received “compassionate use” requests to allow the two missionaries to receive more of the experimental treatment in Atlanta. The agency did say, however, that it “stands ready” to work with companies and health workers “treating these patients who are in dire need of treatment.”

The FDA said it is also working with other federal agencies such as the NIH to fund development of Ebola treatments and to fast-track promising therapies. The agency noted that earlier this year it fast-tracked an investigational Ebola drug developed by Tekmira, a Vancouver, B.C.-based company that had collaborated with the Defense Department. But the FDA recently halted trials for the drug, seeking more information about its safety.

While the outlook for the two Americans battling Ebola appears to have brightened, the situation in West Africa has grown more grim.

According to an update Monday from the World Health Organization, 887 people have died in four countries — Sierra Leone, Liberia, Guinea and Nigeria. More than 1,600 people have been sickened by Ebola in the current outbreak.

With the death toll rising, the World Bank on Monday pledged to provide as much as $200 million in emergency funding to help contain the spread of Ebola in West Africa and to improve the public health systems in the affected countries.