Dr. Kent Brantly was being treated at a hospital in Monrovia, Liberia. Brantly, 33, is the medical director for the North Carolina-based group Samaritan's Purse. He had been working with the group in Liberia since October 2013 as part of the charity's post-residency program for doctors, said Melissa Strickland, a spokeswoman for the group.
"We are hopeful, but he is certainly not out of the woods yet," she said.
The highly contagious virus is one of the most deadly diseases in the world. The World Health Organization said the outbreak is the largest ever recorded, killing more than 670 people in Liberia, Guinea and Sierra Leone since it began earlier this year.
Health workers are at serious risk of contracting the disease, which spreads through contact with bodily fluids.
Photos of Brantly working in Liberia show him in white coveralls made of a synthetic material that he wore for hours a day while treating Ebola patients.
The organization's website said he had worked as a family practice physician in Fort Worth, Texas.
There is no known cure for Ebola, which begins with symptoms including fever and sore throat and escalates to vomiting, diarrhea and internal and external bleeding. Early treatment improves a patient's chances of survival.
"Dr. Brantly recognized his own symptoms and began receiving treatment immediately," Strickland said.