The author argues that contagious disease is not transmitted through the open atmosphere, reviews historical and experimental claims of airborne contagion, and critiques interpretations of sickness among attendants, visitors, and relocated patients. He examines air vitiation from decomposing vegetation and a range of environmental and social causes of pestilence, including marsh effluvia, famine, cold, inadequate clothing, and depressed morale. Later chapters consider limits to contagious spread, compare contagion with fermentative agents, and offer practical public-health measures — ventilation, sanitation, proper disposal of the dead, sewer and housing improvements, adequate diet and clothing, and steps to maintain an active, cheerful mind to prevent and mitigate disease.