Seven years after the US Civil War and nearly 150 years before COVID, horses near Toronto began falling deathly ill with a sickness soon diagnosed as influenza. It took just a few weeks for this mutant flu variant to spread throughout southeastern Canada and into the northeastern US, infecting more than 90 percent of horses, donkeys, and mules wherever it struck. By the time the outbreak relented more than a year later, the Great Horse Flu of 1872 to 1873 had convulsed nearly every corner of North America and parts of Cuba, Mexico, and Central America.

This little-known scourge paralyzed the continent’s horse-powered economy at a critical political moment. As hundreds of thousands of animals died and entire cities ground to a halt, the outbreak exposed the fragility of industrial capitalism and the US’s deep dependence on animal labor. The Great Horse Flu triggered catastrophes including the Great Boston Fire; unleashed seething social and racial conflict; stoked partisan divides; and set the stage for the Panic of 1873. In the hands of Bancroft Prize–winning historian Thomas G. Andrews, the gripping story of this animal plague becomes a revelatory history of American Reconstruction itself—its possibilities, limitations, and demise—while also illuminating the grave perils that novel viral variants pose to animals, humans, and the world we share.