See how a small group of doctors are fighting to save Kylie and others suffering from mystery illnesses on tonight's "AC360" 10 p.m. ET

(CNN) -- Having three daughters of my own, I was a little nervous to meet 6-year-old Kylie McPeak. She was born just about a year before my oldest daughter and was the picture of health up until a couple of years ago.

First, she developed Type 1 diabetes, which she was able to control with medication and her parents' help. But then, little things "just weren't right," her mom, Gina, told me. Her voice might quiver a bit, or her right eye would twitch for a long time.

"People told me I was being overly sensitive," Gina said, but she had a "feeling" that something was off. And, as most doctors know, you should never question a mother's intuition.

Eventually, she and her husband, Steven, videotaped Kylie every chance they got, and it became clear that Gina wasn't an oversensitive mother. Kylie was twitching more frequently now, and her entire right side would start to writhe intermittently. Her voice, which had been previously smooth and clear, now quivered and quaked with every sentence.

Armed with their evidence, they walked into Kylie's doctor's office to get a diagnosis and treatment. The problem was, their doctor didn't know the answer and, in fact, had never seen anything quite like this. That was the same answer they got at the next doctor's office, and the one after that. All the while, Kylie was getting worse.

Unfortunately, thousands of patients are in the exact situation every day. Sally Massagee, 53, had been told something similar. The mother of five had developed the unusual problem of looking like a bodybuilder. Every muscle in her body was ripped.

The problem was, she didn't lift weights, and in case you were wondering, she didn't take steroids. Yet every muscle, including the muscles around her eyes, was getting bigger, and she was starting to look like the Incredible Hulk. It wasn't just a cosmetic problem; it was causing awful pain. And, slowly, her muscles were starting to crush her from the outside in. Yet no one had any answers.

Elite medical unit is last hope for patients with mystery illnesses

For Kylie and Sally, their place of last hope was in Bethesda, Maryland, at a place called the UDP, the National Institutes of Health Undiagnosed Diseases Program. Only a few are chosen to be accepted into this program, and often, doctors are looking not just for patients with rare problems, but also, a diagnosis that could advance science.

After a week of extensive testing, clinical exams by teams of the best doctors in the country and lots of "huddling" among the specialists, the patients finally get to go home. After that, the real work starts: Poring over every piece of information about Kylie and Sally, no matter how small or insignificant it may seem.

Sometimes, an answer emerges to explain why a 6-year-old girl with Type 1 diabetes starts to writhe uncontrollably on her right side and has a voice tremor, or why a woman in her 50s is bulking up without any apparent reason.

This week, you will learn what the doctors found. In the meantime, any idea of what could be causing the symptoms of Kylie McPeak or Sally Massagee?

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