In the Atlantic world between the mid nineteenth and early twentieth centuries, efforts to combat epidemics and reshape urban environments sparked debates over the balance between individual rights and the public good. In Constitutional Epidemic, Pedro Jimenez Cantisano focuses on Rio de Janeiro, where expanding administrative powers treated the poor as obstacles to public interests. In the name of modernization, officials forcibly vaccinated, arrested, and evicted residents, disinfected homes, and expropriated properties. People from diverse social and racial backgrounds drew on colonial, liberal, and progressive legal traditions to shape public policy. In a post-emancipation city shaken by scientific progress, judicial battles, and a violent popular revolt, the public good ultimately prevailed. At the same time, the poor – predominantly Black people and immigrants – asserted their place in legal debates, expanding the boundaries of citizenship. Their words and actions helped shape Brazilian constitutional politics throughout the remainder of the twentieth century.