Doctors are struggling against an Ebola outbreak in Sierra Leone as fears grow that the disease might spread globally. (Reuters)

Kent Brantly and Nancy Writebol, the Americans who contracted Ebola in Liberia, are both in serious condition and are expected to be evacuated to the United States in the next few days, the Christian aid group Samaritan’s Purse said Friday. They are to be taken by private “air ambulance” to a military airfield outside Atlanta, then transferred to a special containment unit at Emory University Hospital.

Brantly, 33, an American physician and father of two based in Fort Worth, traveled to Liberia in October for a post-residency placement with Samaritan’s Purse. When the outbreak began, he ended up directing the Ebola clinic at the Eternal Love Winning Africa hospital in the Liberian capital of Monrovia.

His wife, Amber, and their children had been living with him but returned to the United States before he exhibited symptoms. They are under a 21-day “fever watch,” because the virus can potentially take that long to sicken an infected person.

“I remain hopeful and believing that Kent will be healed from this dreadful disease,” Amber Brantly said in a statement Thursday. She added that she and the children left Liberia before Brantly was exposed to the virus and were physically healthy, but they were anxiously awaiting news from Liberia on Brantly’s condition every day.

“During our time in Texas, the children and I have enjoyed the reunion of family. Our kids have been a welcome relief and distraction to us all, reminding us of our joy and hope,” she said.

The Ebola outbreak that emerged in March in West Africa has killed more than half the 1,300-plus people who have been infected, making it the deadliest outbreak ever. The virus, which causes severe bleeding and has no known cure, has been found in Guinea, Sierra Leone and Liberia.

Writebol joined the mission organization SIM last year with her husband, David. The couple, from Charlotte, have two adult sons who live in the United States. The Writebols were based in Liberia before the outbreak began and stayed to help battle the epidemic. Nancy Writebol worked as a personnel coordinator and assisted the nursing staff at the ELWA hospital.

Another American citizen, ­Liberian-born Patrick Sawyer, died of Ebola in Lagos, Nigeria, on July 25. Sawyer, a consultant for the Liberian Finance Ministry, collapsed in the Lagos airport after showing symptoms of Ebola on the flight.

Sawyer was planning to visit the United States just weeks later to keep a promise he made to his daughters — that he would be at their birthday parties.