Atlanta (CNN) -- Dr. Kent Brantly answered a calling.
That's what friends and colleagues say about the man who garnered national headlines when he became the first known Ebola hemorrhagic fever patient in the United States.
Brantly, 33, arrived Saturday in Atlanta from Liberia, where he and another American missionary worker contracted the deadly virus while caring for Ebola patients.
From an early age, Brantly was driven by his faith in God to make a difference, friends and former colleagues said. He took mission trips to Uganda, Honduras, Nicaragua, Tanzania and Haiti, they said.
"He intended to be a missionary before he became a doctor," friend Kent Smith, an elder at Southside Church of Christ in Fort Worth, Texas, told CNN.
"Eventually, he decided medical mission is what he wanted to do."
Brantly went to Liberia with his wife and two children last year to serve a two-year fellowship through Samaritan's Purse post-residency program.
He was there initially to practice general medicine. But when the Ebola outbreak began, he took on the role of medical director for the Samaritan's Purse Ebola Consolidated Case Management Center in Monrovia. It's there that he tested positive for the virus, according to the evangelical Christian relief charity.
There is no known cure or vaccine for Ebola, and it has a mortality rate of up to 90%.
Brantly is in an isolation ward at Emory University Hospital, near the headquarters for the U.S. Centers for Disease Control and Prevention.
Doctor put sick patients first
The news Brantly contracted Ebola has prompted many to ask why would he put himself at risk.
The answer might be difficult for some to understand, his former college and medical school professor wrote in an op-ed published this week in The Indianapolis Star.
"Simply put, he would say that he had been called to care for the patients in Liberia," Richard Gunderman wrote in the newspaper.
None of it has been surprising to Smith, who first met Brantly five years ago at church.
"When he first started coming to our church, he and his wife made it clear ... they were committed to medical missions," he said.
Before heading to Liberia in October 2013, Brantly did his residency at John Peter Smith Hospital in Fort Worth.
'What do I bring to the table?'
"We're kind of proud that there was a hero out there trying to do his best to make life better for other folks under the circumstances," a physician who knows him, Dr. Paul Pepe of Dallas' UT Southwestern Medical Center, told CNN affiliate WFAA this week.
Brantly attended high school in Indianapolis before graduating from Abilene Christian University (Texas) in 2003 and Indiana University's medical school in 2009.
While at Abilene Christian, he spent a summer interning overseas with a program focused on vocational missions experiences, ACU's online alumni magazine reported.
"Everyone here who has been connected with Kent knows him to be someone who is very compassionate, considerate and always upbeat in all he does," the program's director, Dr. Gary Green, told the magazine.
"... Kent's the kind of guy who would weigh benefits versus risk, then try to take himself out of the equation so that he would be thinking, 'What do I bring to the table? Is the risk worth taking because I can benefit so many people?'"
It perhaps may explain why when only one dose of an experimental serum to treat Ebola was made available this week in Liberia that Brantly turned it down and asked that it be given to his colleague.
"I would have been surprised if he had not done that," Smith said.
In an e-mail this week from Monrovia, Brantly told a fellow doctor at John Peter Smith Hospital that he is "terrified," according to The Indianapolis Star.
"I'm praying fervently that God will help me survive this disease," Brantly said in an e-mail to Dr. David McRay, the newspaper reported.
Though Brantly's wife and children had been in Liberia with him, they were in the United States when he became ill.
"Many people have been asking how I am doing," Amber Brantly said in a statement released earlier this week. "The children and I are physically fine."
The CDC has said neither Brantly's wife or his children are symptomatic.
Brantly's wife, parents and sister cried when they saw him on CNN, walking from the ambulance into the hospital, a family representative said on condition of anonymity. His wife, Amber, later said she was relieved that her husband was back in the United States.
"I spoke with him, and he is glad to be back in the U.S.," she said in statement sent to CNN. "I am thankful to God for his safe transport and for giving him the strength to walk into the hospital."
Brantly's wife visited with him from behind a glass wall for about 45 minutes, the family representative said. Kent Brantly was described as being "in great spirits and so grateful."
CNN's Mariano Castillo contributed to this report.