U.S. doctor, missionary with Ebola show improvement

Two American aid workers infected with Ebola in Africa while responding to an outbreak of the deadly virus have shown slight improvement but remain in serious condition, a relief organization said on Wednesday.

Dr. Kent Brantly and missionary Nancy Writebol were part of a team in Liberia from two North Carolina-based Christian relief groups, Samaritan's

Purse and SIM. The groups plan to evacuate nonessential personnel from Liberia as Ebola cases there mount.

"Because of instability and ongoing security issues in the area, Samaritan's Purse is making arrangements for nonessential personnel to leave the country," the organization said in a statement.

There is no known cure for the highly contagious disease, which has killed 672 people in Guinea, Liberia and Sierra Leone since February. The fatality rate of the current Ebola outbreak, the worst on record, is about 60 percent, health officials said.

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SIM missionary Nancy Writebol and her husband David are pictured in this undated handout photo courtesy of InChrist …

Writebol, who helped disinfect the protective suits worn by medical personnel such as Brantly inside the isolation ward at a care center in Monrovia, Liberia, is "fighting through" the disease, her son said on Wednesday.

Jeremy Writebol told NBC's "Today" show that his mother is moving around on her own and receiving a lot of fluids as she is treated in isolation. "She’s working real hard to get through this," her son said.

Ken Isaacs, a vice president at Samaritan's Purse, told CNN it was believed that a local staff member had come to work already infected with the virus. That staffer has since died.

"We think it was in the scrub-down area where the disease was passed to both Nancy and Kent," Isaacs said.

(Reporting by Colleen Jenkins; Editing by Will Dunham)