(CNN) -- Three vials containing an experimental drug stored at subzero temperatures were flown into Liberia last week in a last-ditch effort to save two American missionary workers who had contracted Ebola, according to a source familiar with details of the treatment.

The drug appears to have worked. Dr. Kent Brantly's and Nancy Writebol's conditions significantly improved after receiving the medication, sources say. Brantly was able to walk into Emory University Hospital in Atlanta after being evacuated to the United States last week, and Writebol is expected to arrive in Atlanta on Tuesday.

On July 22, Brantly woke up feeling feverish. Fearing the worst, Brantly immediately isolated himself. Writebol's symptoms started three days later. A rapid field blood test confirmed the infection in both of them after they had become ill with fever, vomiting and diarrhea.

It's believed Brantly and Writebol, who worked with the aid organization Samaritan's Purse, contracted Ebola from another health care worker at their hospital in Liberia, although the official Centers for Disease Control and Prevention case investigation has yet to be released.

A representative from the National Institutes of Health contacted Samaritan's Purse in Liberia and offered the experimental treatment, known as ZMapp, for the two patients, according to the source.
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The drug was developed by the biotech firm Mapp Biopharmaceutical Inc., which is based in San Diego. The patients were told that this treatment had never been tried before in a human being but had shown promise in small experiments with monkeys.
http://edition.cnn.com/2014/08/04/health/experimental-ebola-serum/index.html?hpt=hp_c1