The condition of a North Texas doctor who contracted the deadly Ebola virus while working for a relief organization in Africa is worsening, colleagues with Samaritan's Purse say.

Dr. Kent Brantly spent four years at John Peter Smith Hospital in Fort Worth before moving to Liberia in the fall of 2013 to work with the international relief agency Samaritan's Purse.

Ken Isaacs, a vice president of Samaritan's Purse, told The Associated Press on Sunday that Brantly, the 33-year-old medical director for the group's Ebola care center on the outskirts of the Liberian capital of Monrovia, was stable and in very serious condition.

In the 48 hours since that notification, Brantly's prognosis has changed to grave.

Early Tuesday afternoon, Melissa Strickland, the spokeswoman for the relief organization, said Brantly's condition has worsened and that at first he was able to get out of bed and use his computer, but that he was no longer able to do that.

She added that it is fair to say that both "he and Nancy [Writebol] are fighting for their lives."

Writebol, also an American, is a worker with allied aid group SIM, or Serving in Mission, which runs the hospital where Samaritan's Purse has an Ebola care center on the grounds. She was in stable and serious condition.

Meanwhile, Brantly's family continues to pray that their son will recover from the deadly disease that has infected more than 1,100 and killed more than 660 in West Africa.

"We are maintaining our focus on our faith in God, and asking for prayer for Kent and we're praying without ceasing," said Jan Brantly, Kent's mother. "I pray for all of those who are battling this disease."

His colleagues in North Texas at JPS said Brantly and his wife Amber had been planning for years to participate in missionary work and that Amber Brantly had only just returned to North Texas with their two children a few days before her husband had been diagnosed with Ebola.

"I couldn’t be more sad, and on the other side, I couldn’t be more proud," said Robert Earley, CEO of JPS Health Network. "It’s gut wrenching."

"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.

"This has been a lifelong dream for him to be a missionary in Africa," said Dr. David McRay with the John Peter Smith Hospital Health Network.

Doctor David McRay Talks About His Friend Doctor Kent Brantly who is fighting the Ebola virus.

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.

McRay said Kent Brantly is in the first week of the disease and is showing symptoms associated with the progression of the illness. The two have been able to talk on the phone and email.

He also said Kent Brantly wants everyone to know he knew he went into an area where there are many diseases with no immunizations or cures and that the Brantly family didn't intentionally go into an area where Ebola was detected.

"Ebola arrived in Liberia after the Brantlys arrived," said McRay. "[Kent] followed protocol and the source [of the infection] is unknown at this time."

According to McRay, Brantly has been in isolation since Wednesday. His family flew back to North Texas about a week ago and "they have absolutely shown no symptoms," said Strickland.

"When they were called to Liberia he [Kent] never questioned that, never thought twice about it and answered that call," said Dr. Jason Brewington, who helped supervise Brantly during his four-year residency in maternal health and family medicine at JPS.

Doctor Jason Brewington with JPS Hospital talks about his friend and colleague Doctor Kent Brantly who is fighting the Ebola virus.

On Monday afternoon, Brewington said Brantly is relying on his faith and is a strong man of God. The two are personal friends.

He has deep faith and he is leaning on that, he is leaning on his family, his church family," said Brewington. "Even though he is away on the other side of the world he knows he has a large church family supporting him and praying for him."

"He is a man of compassion, and that shows," said Brewington. "In the middle of this illness, he is asking for prayers not only for himself, but for his co-worker, patients and providers who are caring for him now.

Kent Smith, an elder at Brantly's church, Southside Church of Christ, said if he could pick one word to describe Brantly, it would be "selfless."

"He really does care more about other people than he cares about himself," Smith said. "He's the kind of guy who said, 'Somebody needs to do it, and I have the skills to do it. I'm going to do it.'"

Even while fighting a disease that claims 9 out of 10 lives, Brantly had a special message he wanted to share with everyone on Monday afternoon. He sent it via email to McCray:

What I would like to say is, thank you for all of the prayers and messages of encouragement. I'm praying fervently that God would help me survive this disease. Please continue to pray along with me and pray for my friend Nancy who is also very sick and for the doctors who are taking care of us. Thank you all so much. Peace. Kent.

And in a statement released Tuesday, Amber Brantly said:

"We appreciate so much all the words of comfort and acts of kindness extended to our family. As people with a deep faith in Jesus, we sincerely thank the thousands of people worldwide who have lifted up Kent and this dreadful situation in prayer. We continue to lean on that faith and take great consolation in our God in these times. We continue to believe that God will deliver Kent from this deadly virus. We have a strong family unit within a stronger faith community that has given us incredible support. Kent remains very physically weak but his spirit has been determined throughout this ordeal. This is a challenging time for our family. We will not be speaking to the media at this time. We ask that you respect our privacy. We ask for your continued prayers for Kent, his colleague, Nancy Writebol, and the healthcare workers in Liberia struggling to meet the overwhelming demands of those who are sick with the Ebola virus as well as patients who have come to that hospital with other needs."

Situation in Liberia Growing Worse

Meanwhile, the situation in Liberia continues to worsen, according to a newspaper reporter who spoke to NBC 5 by phone Tuesday.

"People are dying every day," said Alaskai Johnson, a health reporter for the Daily Observer.

He said he met Brantly before the Ebola outbreak.

"Personally, I'm so sad because he was one of the first ones on the front lines," Johnson said.

Brantly is getting intense treatment, but with more and more new victims, help is harder to find, Johnson said.

"Hospitals are beginning to close down because nurses are afraid [to work]," he said.

Now, many victims are dying at home without any treatment.

"It's a disaster," Johnson said. "It's a national emergency."

NBC 5's Jane Geelan-Sayres, Kevin Cokely and Scott Gordon contributed to this report.