Starting in late September 1872, horses started falling ill with a severe respiratory complaint in the countryside about a dozen miles north of Toronto, Ontario. Veterinary experts swiftly diagnosed the malady, which paralyzed street transportation, commerce, and everyday life in Toronto itself during the first weeks of October, as influenza. Over the next year, an equine plague that most contemporaries referred to as the epizootic—and which I call the Great Horse Flu in the book I am completing on this outbreak—spread throughout southern Canada, every reach of the United States, and parts of Cuba, Mexico, and Central America. The novel influenza virus responsible for this outbreak sickened between ninety and ninety-nine percent of horses, donkeys, and mules across this vast swath of the northern Americas.1 Our best guess is that the Great Horse Flu killed between one and four percent of the equines it afflicted—a case fatality rate roughly not unlike those recorded by the Great Influenza Pandemic of 1918–1920 and the COVID Pandemic. In less than a year, an estimated 112,500 to 554,000 horses and ponies perished alongside tens or hundreds of thousands of mules and donkeys.2