Globalization is not new. From the time of ancient migrations, human activities increasingly shaped the ecologies of health and disease around the world. When the peoples of the Western and Eastern Hemispheres encountered each other, the invading Europeans brought their domesticated animals, plants, and diseases with them. These demographic and ecological transformations ushered in a new era for animal healing and veterinary medicine. How were animal diseases circulating around the world due to exploration, colonialism, war, and trade? What was the impact of these diseases on human health and well-being, and on the projects of colonialism and state formation? The impacts of large-scale animal epidemics and pandemics enabled by the ecological exchanges of animals, parasites, and pathogens are analyzed. Further, this chapter highlights the development of physiology, pathology, and new disease causation models, while investigating how medical concepts, popular beliefs, and therapies were used in animal health care.