Public health in Brazil achieved remarkable development at the turn of the 20th century thanks in part to physicians and social thinkers who made it central to their proposals for “modernizing” the country. Public health was more than a set of medical and technical measures; it was fundamental to the project of nation building. I trace the interplay between public health and social ideas in the late 19th and early 20th centuries. Physicians and social thinkers challenged the traditional belief that Brazil’s sociocultural and ethnic diversity was an obstacle to modernization, and they promoted public health as the best prescription for national unity. Public health ideas in developing countries such as Brazil may have a greater impact when they are intertwined with social thought and with the processes of nation building and construction of a modern society.