Transcript for Doctors struggle to diagnose man’s mysterious illness

a horrible, horrible, horrible nightmare. Reporter: Jennifer Gunkel couldn’t have felt more helpless or farther away from her boyfriend, Jonathan Koch. She’s home in L.A. While he’s across the country in Washington, D.C. In intensive care, clinging to life with only a 10% of survival. The doctors were saying, you need to call your loved ones now. Reporter: Like Jonathan, she too was in show business, doing social media shows like like “Grey’s anatomy.” She starts recording, in real time, nearly every moment of this heart-rending, long-distance ordeal. I’m just really scared. Reporter: Overnight, she’d received an ominous text to call him. He said that, things are really serious. This is more serious than I thought. And I said, I’m getting on a plane right now. Reporter: Back at George Washington university hospital, Jonathan’s I.C.U. Doctor, Lynn Abell, is baffled by her patient’s mystifying illness. His heart, his lungs, his kidneys, his liver, his circulation all were failing. Reporter: Agitated, and now wracked with pain throughout his body, doctors fear Jonathan is in full septic shock. Things get so dire, doctors resort to putting in a breathing tube. But first, doctors have to sedate him, him, putting him into what is essentially a medically induced coma. He just begged to have us try to hold off, until his fiance arrived. He was on his near last breath. I wish I could have talked to him. I wish I would have gotten here sooner. Please give me one more kiss or one more hug. Reporter: As agonizing hours turn into days, there were still no answers and Jennifer, his medical proxy has to okay every medical test, every procedure, knowing those decisions could affect his daughter too. Yeah, I just kept thinking about his daughter and just like she can’t be without her dad — and so I was thinking every moment every decision I made was like this little girl. I can’t let this girl lose her father. Reporter: But it’s the pictures that tell the story. And Jennifer is there documenting every moment, capturing the before, not even sure there will be an after. By his second day in the I.C.U., a terrible sign — his limbs are turning black. Gangrene is setting in, because his vital organs are literally stealing the blood from his arms and legs to survive. His fingers are blue and his feet are blue. Reporter: Then, as things are at their most desperate, doctors propose something that, to Jennifer, is unthinkable. They also talked about amputating your fingers and toes. I asked them why. Lisa, his sister, and I, we just turned and looked at each other and we were like, “What? Did he just say amputation?” And we were shocked. I just thought “I will just never make that decision for him.” Reporter: If anyone knew what kind of fighter Jonathan could be, it was his sister, Lisa. After all, their survival story began long before this. I always felt like we were a team, the two of us going through it. Reporter: A team with her brother. Growing up in the Pennsylvania town of state college, home of Penn state. Raised by a loving mother, but a father who Jonathan says had no use for him. My father had a very domineering, almost sadistic way about him. Reporter: Take the time when Jonathan was about four, and he says his father showed him how to tie his shoes just once, then banished him to the basement. And he said, “Go downstairs and don’t come up until they’re tied.” And it took me a long time. Probably, I don’t know, 10 or 12 hours. I could hear my mom crying out the door. Reporter: But Jonathan says there is one gift he credits his dad with giving him. He took Jonathan to a drive-in when he was 11. A transformational movie, called “Rocky.” “Rocky” really changed my life. Reporter: It might seem cliche to come, but not if you’re a downtrodden kid from the keystone state, who’d come to L.A. With dreams of stardom, only to end up a Hollywood heavy weight. You took rocky as gospel. It is the gospel. What are the lessons? Well, first of all, he didn’t win. Rocky didn’t win. But to me, he accomplished what he set out to accomplish which — he wanted to last the entire fight. To me, that was everything. Reporter: But did Jonathan have enough fight in him to beat whatever mysterious illness had breached that otherwise healthy body? Finally, after two and a half days in the I.C.U., Dr. Seneff says it’s one of the young doctors, a resident making rounds, that has an ah-ha moment. The more detective work we did, the more likely it seemed that this really was the diagnosis. Reporter: A lymph node biopsy confirms it. The appropriate term, I think, is hemophagocytic lymphohistiocytosis. Reporter: Also known as hlh, a rare and aggressive immune disorder that neither of Jonathan’s doctors had ever confronted. It is basically your immune system in overdrive. And there’s some trigger, often times viral, but it’s mostly seen in pediatrics. Reporter: They immediately reverse course on treatment. What they had been doing was making things worse. They start chemotherapy — along with large doses of steroids, to kill the toxins. And over the short course of time, over just a few days, he started making a quick recovery of his organ systems. You opened your eyes three times today which was amazing. I saw the light in your eyes, your beautiful blue eyes. Honey, squeeze your hand again. Reporter: Like the prize fighter he idolized, Jonathan survived the fight of his life. But the damage was done. Gangrene had literally starved his limbs to death. At a minimum, his left hand would need amputation. When you emerge from that coma, what’s the first thing that you think? The first thing I thought was that I wanted to marry Jennifer. And yet before that, you weren’t planning on getting married? No. What changed? All of a sudden it was just about us.

This transcript has been automatically generated and may not be 100% accurate.