An experimental drug made from tobacco plants appears to be saving two Americans infected with Ebola.

The biotechnology drug produced by Mapp Biopharmaceutical Inc., a San Diego company with nine employees, had been tested only on infected animals when it was given to the two health workers, Bloomberg reported.

A subsidiary of Reynolds American Inc. (NYSE:RAI) manufactures the ZMapp drug.

The patients, Christian missionaries Dr. Kent Brantly and Nancy Writebol, each received a dose of ZMapp in Liberia before flying to the U.S. for isolated treatment in Atlanta.

Last Thursday, Brantly told his doctors in Liberia he was dying, as his breathing became labored on the ninth day of his symptoms. But within an hour of the first dose, his condition reversed, causing a doctor to call his recovery "miraculous," according to CNN.

In trials, small monkeys were given the drug within 48 hours of infection with Ebola. One monkey not given the drug died on the fifth day of infection.

Brantly arrived at Emory University's hospital on Saturday. After receiving a second dose of the drug, his condition has continued improving, according to Samaritan's Purse, his mission organization. Writebol returned to the U.S.

Tuesday in “serious, but stable, condition,” according to her aid organization SIM. She will be given a second dose of the drug upon arrival to Atlanta.

ZMapp hasn't gone through clinical trials, so its administration to Brantly and Writebol, though with their consent, is highly unusual.

Though the drug is not difficult to manufacture, pharmaceutical companies don't have an economic incentive to produce it because developing and licensing the drug would be costly, and Ebola has killed only thousands since 1976, mostly poor Africans, health workers told the Telegraph.