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."

NBC