Kathleen Seidel is a New Hampshire mother of two. She runs a blog, Neurodiversity.com, about autism--a topic of special interest to her as one of her children had an "autistic spectrum diagnosis." There is a theory that thimerosal, a preservative formerly used in vaccines, causes autism. Seidel has written skeptically of that theory, and of the trial lawyers who have sought to cash in on it.

Last month one of those lawyers, Clifford Shoemaker, had Seidel served with a subpoena demanding that she report for a deposition and...