SAN DIEGO (CBS SF) — An experimental serum developed by a California company may be keeping an American doctor who became infected with the Ebola virus in Africa alive.

A couple of days ago, Dr. Kent Brantley took a turn for the worse: he developed a rash all over his body and he couldn’t breathe. Then doctors gave him a serum that had never been tested on humans before.

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When Brantley stepped out of the ambulance at Emory University in Atlanta over the weekend, it was clear the serum was working.

“We all assumed that he was going to be rolled out on a gurney. To see him walk, absolutely, a renewed sense of energy and hope,” said Kent Smith, a church elder who knows Brantley.

The American doctor became infected with Ebola while doing missionary work in Liberia.

Before he left West Africa, he received a dose of a special serum known as ‘ZMapp,’ created by a San Diego-based research firm called Mapp Biopharmaceutical.

The serum contains antibodies that prevent the virus from entering cells in the body.

“There are very, very few doses and apparently the company is trying to scale up. It’s not easy to scale up to very large numbers of doses, but that’s something that is under intense discussion now,” said Dr. Anthony Fauci of the National Institutes of Health.

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While ZMapp has not been tested on humans, it has shown promising results in monkeys.

Three precious vials of the medicine, stored at subzero temperatures, were flown to Liberia last week. They were administered to both Brantley and fellow missionary worker Nancy Writebol, who was also infected.

Doctors say soon after the doctor was given the drug, his condition improved dramatically.

“We need to be careful. I do hope that it was as impressive as being described, because if it is, that bodes very well for that particular product,” Fauci said.

On Monday, Brantley remained in a state of the art isolation unit, receiving round-the-clock care.

His wife has been able to visit him, but they can’t have physical contact. She had to talk to him through an intercom, from the other side of a thick glass panel.

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At least 887 people have died from Ebola in Guinea, Sierra Leone, Nigeria and Liberia.

Elizabeth Cook