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.