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.