The Black Death, often called the largest pandemic in human history, is conventionally defined as the massive plague outbreak of 1346 to 1353 C.E. that struck the Black Sea and Mediterranean, extended into the Middle East, North Africa, and western Europe, and killed as much as half the total population of those regions. Yet genetic approaches to plague’s history have established that Yersinia pestis, the causative organism of plague, suddenly diverged in Central Asia at some point before the Black Death, splitting into four new branches—a divergence geneticists have called the “Big Bang.” Drawing on a “biological archive” of genetic...