The fourth chapter, “Innovations,” discusses the environmental challenges in a rapidly urbanizing London, the capital of the largest empire of the modern period. It explores the early innovations in dealing with excreta disposal, including the creation of an underground sewer system and efforts to use the highly dilute sewer effluvia as fertilizer. The direct health benefits of modern sewerage alone were modest. Many smaller and less wealthy cities and towns opted for other methods of human waste disposal, including the tub-and-pail system. Much infectious intestinal disease was the result of pathogen-laden flies alighting on food and the contamination of the urban milk supply. The major reductions in mortality and morbidity from intestinal pathogens came about as a result of the filtration and chemical treatment of drinking water with chlorine or ozonation.

Chapter three, “Diffusion and Amplification,” discusses the long era in which pathogens and parasites were extended to new regions. As human communities became more complex, networks of trade expanded and became denser, allowing for the rapid, long-distance transmission of intestinal pathogens. Over the first millennium and a half of the Common Era, the disease pool of Eurasia and northern Africa became increasingly integrated. In the late fifteenth century, some Old World intestinal pathogens crossed the Atlantic and became established in the Americas. By the early nineteenth century, the integration had become global. Rapid urbanization in the industrializing North Atlantic states created a crisis of urban fecal pollution. In response, the first public health reform movements emerged. Beginning in the first half of the nineteenth century, cholera pandemics spread along global trade routes and infected all the inhabited continents. This provoked the first efforts at the international control of disease.