A miasma is a poisonous atmosphere formerly thought to rise from swamps and putrid matter and cause disease.  Miasmas were for a long time equaled to stench. Thus sewers, but also peasants and common workers, who could not afford to bathe regularly were associated with diseases. Miasma was thought to be the cause of diseases like pest, cholera and typhus. The idea of the miasma was disproved by the emergence of the bacteriologists like Robert Koch and Louis Pasteur.
Corbin, Alain: Pesthauch und Blütenduft. 2005