Veterinary education, training, and employment shifted to support military needs in wartime. Conflicts around the world, including World War I, relied on millions of horses, dogs, and food-producing animals to supply armies. Wartime disruptions, and the movement of so many animals, sparked outbreaks of diseases that challenged animal owners, healers, and veterinarians. The use of horsepower declined in industrialized areas, depriving veterinarians of their most important patients. Many turned instead to livestock and food production. National campaigns against bovine tuberculosis, brucellosis, and other zoonoses employed many veterinarians. Others worked on vaccines and therapeutics in biomedical research. With the outbreak of World War II, ethical questions troubled veterinarians who contributed to the development of biological weapons. Rebuilding the world’s food production systems after the war stimulated international veterinary cooperation and incorporated new tools, such as antibiotics. Veterinarians also helped make intensive animal production ("factory farming") possible by controlling diseases, while more and more vets in wealthier areas treated companion animals (pets).