Kristie Tunick, 32, (pictured here in December 2007) was an extremely active young woman, happily planning her wedding to fiance, Brian Mendenhall -- when her life was turned upside down. Suddenly, Tunick, who had been a dancer and athlete, could barely move. Her muscles began to lock up, she had trouble breathing and digesting, seemingly without explanation. Tunick went in and out of hospitals trying to get a proper diagnosis -- and to this day, the exact nature of her illness remains a mystery. Doctors believe she may suffer from stiff person syndrome, or SPS, a one-in-a-million auto-immune disorder, but her dramatic, evolving symptoms continue to baffle.
Watch "20/20" FRIDAY at 10 p.m. ET for Tunick's story and more medical mysteries (Courtesy Brian Mendenhall and Kristie Tunick)