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.