Anti-Ebola Serum Being Tested on US Patients.

Two Americans who contracted the deadly Ebola disease while working in Africa seem to be improving after being treated with an experimental biotechnology drug that is produced from tobacco.

U.S. health authorities have campaigned for years against smoking and using tobacco products to cut down on cancer deaths. But  Mapp Biopharmaceutical, a California company with only nine employees, developed the anti-Ebola serum it calls ZMapp from therapeutic proteins found inside tobacco plants.

The company had seen favorable data from testing the drug on Ebola-infected monkeys, but it had not been sanctioned by U.S. health officials for use on humans.

The company, however, released it for use by the two Americans after they contracted Ebola in Liberia while working there for a Christian charity. The drug was administered to Kent Brantly, a doctor, and Nancy Writebol, a missionary, before they were flown back to the United States for treatment in an isolation tent at a hospital in Atlanta, Georgia.

Officials say the health of Brantly, who arrived Saturday, appears to be improving. Writebol returned Tuesday to the United States and her son Jeremy Writebol says she also is showing favorable signs.

"My dad says she was weak but showing some good signs and some improvement. She was eating a little bit, able to take in fluid," Writebol said. "So, you know, we're cautiously optimistic, feel like this is going to be a good thing for her and we're prayerful that she'll pull through on this."

It is unclear how many more doses of the experimental drug the two may be given. Health officials say there is a "very scarce" number of dosages available, making it unlikely that the drug will be of immediate help in combating the current Ebola epidemic in West Africa.