Nemo the Magnificent writes with this excerpt from the University of Arizona: "A new study (paywalled) published in the journal Nature provides the most comprehensive analysis to date of the evolutionary relationships of influenza virus across different host species over time... In the 1870s, an immense horse flu outbreak swept across North America. City by city and town by town, horses got sick and perhaps five percent of them died. Half of Boston burned down during the outbreak, because there were no horses to pull the pump wagons. In the West, the U.S. Cavalry was fighting the Apaches on foot because all the horses were sick... The horse flu outbreak pulled the rug out from under the economy.""