In 1999, when Americans watched as Michael J. Fox — beloved to many as the smooth, wisecracking Marty McFly — swayed and shifted uncontrollably, clenching and unclenching his fists as he provided solemn testimony to the U.S. Senate Appropriations Subcommittee on the realities of life with Parkinson’s, it drew attention to a disease that affects nearly 1 million people in...