If it’s true that you are what you eat, then most beef-eating Americans consist of a smattering of poultry feathers, urine, feces, wood chips and chicken saliva, among other food items. As epidemiologists scramble to figure out how dairy cows throughout the Midwest became infected with a strain of highly pathogenic avian flu — a disease that has decimated hundreds of millions of wild and farmed birds, as well as tens of thousands of mammals across the planet — they’re looking at a standard “recycling” practice employed by thousands of farmers across the country: The feeding of animal waste and...