In the decades after Hernán Cortés invaded Mexico, one of the worst epidemics in human history swept through the new Spanish colony. A mysterious disease called “cocolitzli” appeared first in 1545 and then again in 1576, each time killing millions of the native population. “From morning to sunset,” wrote a Franciscan friar who witness the epidemic, “the priests did nothing else but carry the dead bodies and throw them into the ditches.” In less than a century, the number of people living in Mexico fell from an estimated 20 million to 2 million. “It’s a massive population loss. Really, it’s...