Drawing on the approaches of intellectual and social history and the work of Michel Foucault, the author investigates the intersection of scientific, political, and professional interests that informed perceptions and understandings of contagion in nineteenth-century France. By charting the development of the modern notion of contagion in France - from the highest echelons of scientific research in the Academy of Medicine to the activities of government authorities to the work of neighborhood hygiene commissions in Paris - the author reveals how the preoccupation with disease was mediated by an attempt to expand the possibilities of government intervention into urban and industrial life, especially life among the working poor.