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A dose of “experimental serum” arrived in Liberia to be tried on a U.S. charity worker struggling for her life — but there was only enough for one of the two infected workers, so Dr. Kent Brantly asked that it be used on his colleague, the group Samaritan’s Purse said Thursday.

Dr. Brantly, a doctor with the group who was also infected, tried an alternative treatment, using blood transfused from a young survivor of the virus.

“Yesterday, an experimental serum arrived in the country, but there was only enough for one person. Dr. Brantly asked that it be given to Nancy Writebol,” Franklin Graham, president of Samaritan’s Purse, said in a statement. “However, Dr. Brantly received a unit of blood from a 14-year-old boy who had survived Ebola because of Dr. Brantly’s care. The young boy and his family wanted to be able to help the doctor that saved his life.”

Nancy Writebol's son, Jeremy, told NBC News he strongly commended Dr. Brantly's selflessness.

“Dr. Brantly has demonstrated once again how Jesus sacrificed for us," he said. "We pray for Kent’s full recovery and healing.”

The group didn’t say which drug had been shipped. There are several in development, including one that a Canadian company had been testing in people already but had stopped while it addressed U.S. Food and Drug Administration concerns.

"There was only enough for one person. Dr. Brantly asked that it be given to Nancy Writebol.”

"All we know and can say it is an experimental drug among several options that was the most promising," said Bruce Johnson, president of Serving in Mission USA. "Doctors on the ground in Liberia with Samaritan’s Purse and SIM were involved in the decision-to-use process, along with the consent of the patients."

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said he also had no idea which treatment the group may be using.

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"The sad fact is we don't have any proven treatment for Ebola. We don't have any proven vaccine against Ebola," he told NBC News.

Companies may provide experimental drugs for use on a compassionate basis in cases of dire need. In this case, U.S. regulators may not need to become involved because the patients are not in the United States.

Some of the drugs are based on antibodies that are produced naturally by the body during infection. In some infections, antibodies from a survivor can help a patient fight infection. But Thomas Geisbert of the University of Texas Medical Branch, who is working to develop both drugs and vaccines for Ebola, said that is not a tried-and-true approach.

"It is a very controversial topic if you are talking about the serum from a survivor," Geisbert said. His team tried it in monkeys and it did not help them.

"This was something that was done on the past for infectious disease," Frieden said. "There are so many things we don’t know about why somebody may recover and which antibodies might be effective."

Graham said his organization was putting safety first.

“The safety of our staff is a top priority and Samaritan’s Purse is currently working to evacuate all but the most essential personnel to their home countries,” Graham said.

“The evacuation should be completed this weekend. The exact timeline and destinations are being kept confidential to respect their privacy. Samaritan’s Purse is taking precautions that exceed the standards recommended by the Centers for Disease Control.”

Samaritan's Purse said Brantly and Writebol were still in grave condition.

The World Health Organization says 1,323 people have been infected with Ebola in the current outbreak and it has killed 729 people. WHO says it is still out of control in some places and people are carrying it across the borders of three neighboring West African countries: Liberia, Guinea and Sierra Leone.

Grantly and Writebol are in Liberia with a team of charity workers from Samaritan's Purse and Serving in Mission (SIM). The affected countries have only the most basic of health care systems and most of the outside medical aid is being provided by these groups, along with Medecins Sans Frontieres (MSF or Doctors Without Borders) and academic teams allied with the World Health Organization, the U.S. CDC or those doing research.

Various labs have been working for decades to come up with drugs and vaccines to fight Ebola and its close cousin, Marburg virus.

One drug made by a Canadian company called Tekmira, with funding from the U.S. government, uses small bits of genetic material called RNA that attach to the virus and target it for destruction by the immune system.That's the one whose testing is on temporary hold.

Further along in human testing is a drug called favipiravir, or T-705. Approved under the brand name Avigan in Japan, it’s being tested for influenza. But early tests in mice suggest it might also work against Ebola.

Another drug called BCX4430 is made by a small biotech company called BioCryst. It seemed to protect monkeys when given as long as 48 hours after they were infected.

Other drugs are based on monoclonal antibodies, engineered immune system proteins that can home in specifically on a microbe. One collaboration grows them in genetically engineered tobacco plants.

The treatment, called MB-003, provided 100 percent protection to monkeys when given right after exposure to the virus, and helped even after symptoms developed.