Answer:
Miasmas [greek- pollution] is a nasty smell or the mist hovering over a swamp. These smells or 'miasmas' were until well into 19 C thought to be how diseases were spread. The open sewers of the towns and cities were quite rightly blamed, but it took a long time to work out that it was not the awful miasma [smell] but the sewage leaching into the water supply that was the problem.