FORT WORTH, Texas >> Kent Brantly always wanted to be a medical missionary, and he took the work seriously, spending months treating a steady stream of patients with Ebola in Liberia.

Now Brantly is himself a patient, fighting for his own survival in an isolation unit on the outskirts of Monrovia, Liberia, after contracting the deadly disease.

The Texas-trained doctor says he is "terrified" of the disease progressing further, according to Dr. David Mcray, the director of maternal-child health at John Peter Smith Hospital in Fort Worth, where Brantly completed a four-year residency.

"I’m praying fervently that God will help me survive this disease," Brantly said in an email Monday to Mcray. He also asked that prayers be extended for Nancy Writebol, an American co-worker who also has fallen ill.

Brantly "went into Ebola exhausted" from treating Ebola patients, Mcray said after speaking with him Monday. His prognosis is grave and efforts to evacuate him to Europe for treatment have been thwarted because of concerns expressed by countries he would have to fly over en route to any European destination, Mcray said.

There is no known cure for Ebola, which begins with symptoms including fever and sore throat and escalates to vomiting, diarrhea and internal bleeding. The disease spreads through direct contact with blood and other bodily fluids as well as indirect contact with "environments contaminated with such fluids," according to the World Health Organization.

Still, colleagues and family members said Brantly, 33, knew of the risks associated with working in one of the world’s poorest countries during an epidemic and did not regret his choice.

"Kent prepared himself to be a lifetime medical missionary," said his mother, Jan Brantly. "His heart is in Africa."

Last October, Brantly began a two-year fellowship with Samaritan’s Purse, a Christian aid group, to serve as a general practitioner, delivering babies and performing surgeries at a mission hospital in the Monrovia suburb of Paynseville.

When Ebola spread from neighboring Guinea into Liberia, Brantly and his wife, Amber, re-evaluated their commitment, but decided to stay in West Africa with their children, ages 3 and 5.

Brantly directed the hospital’s Ebola clinic, wearing full-body protective gear in the Equatorial heat for upward of three hours at a time to treat patients.

He undertook humanitarian work while studying medicine at Indiana University, working in impoverished, inner-city neighborhoods, according to a medical school spokeswoman.

During his four-year family medicine residency, he accompanied Mcray on medical missions to Uganda and earthquake-devastated Haiti. He also spent several weeks working in Tanzania, where a cousin lives and works as a medical missionary, Mcray said.

Before contracting Ebola, Brantly and his family "really enjoyed Liberia."

"They were very well-adjusted," said Ken Kauffeldt, the country director for Samaritan’s Purse in Monrovia.

Liberia’s health ministry is investigating how Brantly contracted the virus.

"We’re trying to figure out what went wrong because he was always very careful," said Tolbert Nyenswah, an assistant health minister in Monrovia.

Amber Brantly and the children departed for a wedding in the U.S. just days before Brantly fell ill and quarantined himself.

They are currently staying with family in Abilene and, while not subject to quarantine, are monitoring their temperatures for an early sign of viral infection, a City of Abilene spokeswoman said.

Their return has sparked questions about whether they might introduce the infection to the U.S.

However, Stephan Monroe of the Centers for Disease Control and Prevention said Monday that "Ebola poses little risk to the general U.S. population."

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Associated Press writers Betsy Blaney in Lubbock, Texas, and Ken Kusmer in Indianapolis contributed to this report.