Time: On the way back from a morning run up the mountain on Koh Phangan island, Natalie Revie felt something was wrong. It was not just fatigue. “I felt my back had collapsed. Any strength or power I had there was gone completely,” recalls Revie, a freelance writer from England who lives in Thailand. “I felt my legs and hips belonged to a dingly, dangly scarecrow.” Revie, 30, stayed in bed the rest of the day, but she went from bad to worse. Her bones were aching all over, and she was …