Clear skies, fresh air, that must mean there are no diseases here? For millennia people believed that the spread of diseases was connected to the quality of the air. If the air was cold, polluted and smelt rotten you were sure to get infected with cholera or the black plague if you were out too long. Miasma theory had an enormous impact on people's habits and even the economy of southern China. It was only in the 19th century that this medical belief was seriously challenged, which led to more effective ways of dealing with pandemics and the prevention of many deaths. For tips on hygiene and disease control listen to this week’s episode of Good in Theory.