Hard cases: Investigating rare & tough diseases Sally Massagee looked like a bodybuilder, but she wasn't. Rather, a mysterious affliction was causing Sally's muscles to grow out of control, and it threatened to affect her heart. After stumping all the experts, Sally finally ended up in the care of Dr. William Gahl at the NIH's Undiagnosed Disease Program -- the last, best hope for people like Sally with rare, debilitating, and mysterious ailments. Lara Logan reports on doctors and patients facing off against unknown illnesses. The following script is from "Hard Cases" which originally aired on May 20, 2012. Lara Logan is the correspondent., producer. Every day across America, people wake up suffering from symptoms of diseases that have never been identified. There are thousands of illnesses passed from generation to generation -- genetic diseases -- about which doctors know next to nothing. But inside the National Institutes of Health -- the government's premier medical research facility in Bethesda, Maryland -- there's a small group of scientists devoted to unraveling the mysteries of the human body and finding cures. The doctor in charge, William Gahl, takes on only the hardest of the hard cases. William Gahl: There's -- a really significant percentage of the American population that has a rare disease. Lara Logan: Millions of Americans. William Gahl: We are talking about millions of me-- Americans. There's no question about that. Dr. William Gahl is the tireless force behind the Undiagnosed Diseases Program, investigating illnesses so rare they don't appear in any medical book. Lara Logan: What defines a rare disease? William Gahl: A rare disease in the United States is one that affects fewer than 200,000 individuals out of the 350 million citizens in the United States. That would be rare by legal definition. Lara Logan: By your definition. Cause you have a different definition? William Gahl: Something that affects 200,000 people isn't all that rare to us. I mean, when we're talking about rare diseases, the disorders were affecting between one and 50 people in the world. Christine Davidson: Uh, I'm so tired. 45-year-old Christine Davidson is one of those people. She's been struggling with an undiagnosed condition for more than half her life. Christine Davidson: I kinda feel like my muscles are just torturing me. Yeah, my spasm in my hip squeezes so hard it just pops my bones. Lara Logan: They are torturing you. Christine Davidson: Yeah. Whatever is wrong with Christine has left her unable to walk and gradually taken over her body. Christine Davidson: I have, like, frozen shoulder syn-- like, I can't really lift my arm. And I have restrictive lung disease where the muscles are tightening so much I only breathe, like, 50 percent of what a normal person would breathe. After years shuttling from one specialist to another, she traveled across the country from her home near Seattle to see Dr. Gahl. Christine Davidson: I'm just getting to the point where I'm really at my last leg. So this chance to come here to be evaluated was really a big deal for me.