Librarian Susanne Caro was leafing through an 1888 book on Civil War medicine when she spied a small, yellowed envelope tucked between the pages. Freeing it, she read the inscription "scabs from vaccination of W.B. Yarrington's children" in the corner, with the signature "Dr. W.D. Kelly," the book's author. After some research, the 23-year-old Santa Fe, N.M., woman decided not to open the envelope. "The only thing I could find connected with it," she said, "was smallpox." After a cross-country relay involving the FBI, the National Museum of Civil War Medicine in Frederick, Md., and Washington's Walter Reed Army Medical...