Right up until the mid 1800s (around 1840) it was thought that Cholera and so on were caused by the smell in the air (the miasma). Nobody knew about germs carrying the diseases.
In fact to try to cure the Cholera problem people like Joseph Bazalgette wanted to improive the London sewers to get rid of the smell / miasma which they thought would get rid of Cholera.
When the London sewers were improved the water supply improved so it DID get rid of Cholera, but not for the reason they thought.
Read more about Bazalgette and the "great stink" here
http://en.wikipedia.org/wiki/Joseph_Bazalgette
http://en.wikipedia.org/wiki/The_Great_Stink