European Medical thought on epidemics - ranging from the opposition clean/dirty, pollution, malaria, pestilential seeds to 19th century microbiology - has always been ruled by figures such as the miasma, the repulsive, the excrement, the filth, the corrupt, the contagion. Theoretically framed, in general, by Foucault's ideas, this research focuses on some aspects of the historical epistemology and ethics of Public Health. Based on the history of Tuberculosis (TB) management in Portugal and, more briefly, on the flu pandemics, we thematize aspects of Biopolitics, namely, the disciplinary power and the processes of indifferentiation and reification. We will focus on the disjunction between contamination and transmission, on the systems of inclusion and exclusion and on the differences between uncertainty, risk, danger, precaution and prevention, aiming to contribute to the public debate on an ethic of precaution. It will be attested that the praxis of Public Health inevitably calls together, at various levels, the old categories of medical ethos, among which are the catharsis, the crisis, and the kairos.