The seventh chapter, “An Era of Optimism,” analyzes the new culture of sanitation practices that helped to define modernity. In the late nineteenth and early twentieth centuries, those living in the developed world became accustomed to wearing shoes, using toilet paper, bathing regularly with soap, and utilizing refrigeration systems to extend the life of foods. In the mid-twentieth century, populations in the Global North benefited from population-wide vaccination programs against poliomyelitis, the prevalence of which seemed to have increased as a result of the implementation of better sanitation systems. Based on the "hygiene hypothesis," many specialists believed that poliomyelitis was rare in regions without modern sanitation. This was not the case. Regrettably, polio vaccination did not begin in the developing world until the 1970s. Oral rehydration therapy, a major breakthrough in the treatment of diarrheal disease, saved millions of lives.

The conclusion summarizes the major themes and findings of the book. The first major advance in the control of infectious intestinal disease in the modern era was in the treatment of water supplies. In the first half of the twentieth century, the combined package of underground sewerage and purified water won broad cultural acceptance. Modern sanitation conveyed enormous population-level benefits, even as it produced some unanticipated vulnerabilities and contrary health outcomes. Oral rehydration therapy and childhood immunizations have dramatically improved childhood survival rates, contributing to soaring population growth and deepening environmental challenges.

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.

The second chapter, “Early Change,” explores what is known about environments in which intestinal disease transmission emerged. It marshals research in the biological sciences to discuss the settings in which early communities were able to transmit some intestinal pathogens and parasites, long before the agricultural revolution. It suggests that the construct of the “first epidemiological transition” needs to be revised. It explores the patterns of vulnerability to infectious intestinal disease associated with hunting, gathering, and fishing in an early era and those associated with early farming practices, settlements, and pastoral nomadism. It provides a historical overview of the evolution of zones of infectious intestinal disease, the various Eurasian attitudes toward human waste, regional patterns in the use or non-use of human excreta in early agriculture, and early urban sanitation.

The Guts of the Matter is a study of our oldest ecological problem: the transmission of infectious intestinal pathogens from human waste. Over deep time, fecal pathogens have killed innumerable infants and young children and been a principal constraint on human population growth. Over the past several generations, we have gained increasing control over the transmission of infectious intestinal disease. These advances have contributed to an ongoing population explosion that is dramatically transforming global ecological systems. The introduction provides a guide to the book’s structure, introducing the individual chapters with a brief summary of each.

Chapter five, “Adoptions and Adaptations,” explores the evidence for the adoption of modern sewerage and water purification systems beyond the early centers in northern Europe and North America. The principal constraints to adoption of modern sanitation were fiscal, although ecological, political, and cultural forces also played large roles. The overall result was that the sanitation revolution beyond the North Atlantic was adopted piecemeal and that the benefits were generally concentrated in the core urban areas inhabited by those with political and economic power. Into the mid-twentieth century, the flush toilet and the disposal of human waste via water carriage made little impact on the overall problem of excreta disposal, and even the provision of piped water was generally limited to the cities and large towns in which Europeans, local elites of European ancestry, and/or non-European elites had an authoritative presence.

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.

Chapter eight, “Infectious Intestinal Disease and Global Public Health,” explores the major challenges to and advances in the control of infectious intestinal diseases since 1970s, when new sets of actors took up the crusade against diarrheal diseases. Physicians and activists organized in response to a spike in infant deaths in the Global South linked to bottle-feeding. Nongovernmental agencies and national politicians encouraged behavioral change to end open defecation. Biomedical scientists developed additional vaccines against poliovirus and rotavirus. Epidemiologists threw new light on the global prevalence of diarrheal diseases and regional disparities in childhood survival rates. The new focus on biomedical interventions and programs of community- or national-level behavioral change constituted a new era in the control of infectious intestinal disease.

Chapter six, “The Struggle Against Hookworm Disease,” examines the early campaigns of the Rockefeller Foundation to reduce transmission of the widespread helminthic infection. Launched in the southern United States and then extended southward in the western hemisphere and into the eastern hemisphere, the anti-hookworm campaigns became the very first global health initiative. Although the campaigns utilized chemical therapies to reduce the intestinal worm load, their primary focus was on changing defecation habits, to encourage better sanitation. The campaigns failed to meet their goals, underscoring the limitations of mass drug treatment and the difficulties of changing entrenched defecation practices and the use of human waste as fertilizer.

The first chapter, “Pathogens and Parasites,” introduces the major helminthic, protozoal, viral, and bacterial intestinal disease agents, and it provides estimates of their current prevalence and contribution to the burden of human disease. The chapter discusses the biological and social determinants of infectious intestinal disease transmission, and it makes the point that, although a range of hygienic practices can have a significant influence on transmission, owing to a range of ecological and cultural variables, few universal rules apply. It discusses some recent findings from the microbiome project that provide new ways of thinking about infectious intestinal disease, and it makes the case that a deeper understanding of the historical epidemiology of infectious intestinal diseases can potentially improve the public health outcomes from contemporary interventions.