David Quammen's new book is an extended excerpt of his previous one, Spillover, which explored how dangerous pathogens jump from animals to people. Courtesy of David Quammen hide caption

itoggle caption Courtesy of David Quammen

Former President George W. Bush (right) and his father, former President George H.W. Bush, wave as they leave a family wedding in Washington, D.C., in May 2006. Normand Blouin-Pool/Getty Images hide caption

itoggle caption Normand Blouin-Pool/Getty Images