By most accounts, the history of AIDS begins sometime in the late 1970s, before the first official cases were diagnosed in 1981 among a handful of gay men. But a striking new book by Dr. Jacques Pépin, an infectious disease specialist at the University of Sherbrooke in Quebec, upends medical history. In “The Origins of AIDS,” Pépin traces the roots of the disease back to 1921 when a handful of bush-meat hunters in Africa may have been the first to be exposed to infected chimpanzee blood. Donald G. McNeil, Jr., science reporter for The New York Times, reviewed the book, and talks about the surprising origins of the world’s most devastating pandemic.