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In Antiquity the influence of air on health was considered obvious. Air was conceived of as the main cause of disease, either by its seasonal variation (each season having its own characteristic diseases) or by its intrinsic toxicity (for example near marshes). Air was especially used as the main explanatory factor for epidemics, when many people become ill in the same way, which is better explained by the air we all breathe than by individual factors such as food and drink intake or physical activity. While ancient medicine tended to put the emphasis on compensating the influence of air in order to finely tune the balance between our body and its environment, much thought was also given to prevention, particularly in the fields of town planning or agronomy: avoiding places with bad air was crucial when selecting the site for a new city or a new farm, but some strategies were also devised in order to improve air quality and integrate human settlements into an unhealthy environment.