Enormous human disasters have seemed to mark my writing career of the last 25 years. It was tragically, coincidentally, right after September 11, 2001, that I began work on the book I’d just sold to HarperCollins—The Great Mortality: An Intimate History of the Black Death, the Most Devastating Plague of All Time.

I had been drawn to the subject through following an AIDS patient for two years in the early 1990s, when effective H.I.V. treatments were rare, for a book I was writing on experimental medicine. It was then that I saw the power of an unfettered pandemic firsthand, at least as we knew it then: the man lost a former lover, three friends, a work colleague, and two dozen acquaintances in fewer than 10 months. The year before I finished that book, 1994, Jonathan Mann, a professor at the Harvard School of Public Health, presciently warned that “the history of our time will be marked by recurrent eruptions of newly discovered diseases.” Three years later, The New England Journal of Medicine reinforced the prediction: “The threat from emerging infectious diseases is not to be taken lightly.”