"We are hopeful and prayerful," Isaacs told AP by telephone from the group headquarters in Boone, North Carolina. He said the doctor quickly recognized the symptoms and sought speedy treatment.
Isaacs identified the second American, Nancy Writebol, as a worker with an allied aid group SIM, or Serving in Mission, which runs the hospital where Samaritan's Purse has an Ebola care center on the grounds. He said she was in stable and serious condition.
"She is showing full symptoms of the disease," Isaacs said. He added that Writebol had been working as a hygienist who decontaminated those entering and leaving the Ebola care area at the hospital.
He said both Americans have since been isolated and are under intensive treatment.
Isaacs, the Christian relief group's vice president of program and government relations, said the fact that health care workers have been infected underscores the severity of the West Africa outbreak that has killed hundreds in Liberia, Sierra Leone and Guinea.
"It's been a shock to everyone on our team to have two of our players get pounded with the disease," said Isaacs, adding health ministries in those poor nations are challenged to respond. "Our team is frankly getting tired."
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.
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. The WHO says the disease is not contagious until a person begins to show symptoms.
Brantly's wife and children had been living with him in Liberia but flew home to the U.S. about a week ago, before the doctor started showing any signs of illness, said Melissa Strickland, a spokeswoman for Samaritan's Purse.
"They have absolutely shown no symptoms," she said.
A woman who identified herself as Brantly's mother said the family was declining immediate comment when reached by phone in Indiana late Saturday.
Brantly is a graduate of Indiana University School of Medicine and went to Liberia as part of a two-year fellowship with Samaritan's Purse, shortly after he completed his residency in family medicine at John Peter Smith Hospital in Fort Worth, Texas.
"The caliber of a person like that who says, 'I'm going Africa, I'm going to where people need me the most,' it really speaks to you," Robert Earley, president and CEO of JPS Health Network, said Sunday. "It speaks to your heart."
John Munro, pastor of Calvary Church in Charlotte, North Carolina -- which sponsors the Writebols' work as missionaries -- said he told his congregation on Sunday the news.
Munro said Writebol's husband David told an elder in the church via Skype on Saturday that she was very sick and he couldn't even enter the same room with her.
Munro said some church members had offered several months ago to pay to fly the Writebols back to the U.S. because of the Ebola outbreak but they refused because they felt God had called them to work there. He broke the news to the congregation Sunday morning.
"These are real heroes -- people who do things quietly behind the scenes, people with a very strong vocation and very strong faith," Munro said.
He said the couple has worked as missionaries since the 1990s and previously worked in an orphanage in Zambia, adding they left for Liberia just under a year ago. Their hometown wasn't immediately disclosed.