John Snow hears about the recent Cholera outbreak. This was not the first one. He knew that it was not caused by miasma.
It could be caused by something else
CHOLERA BREAKOUT CAUSED BY MIASMA (bad air)
John Snow lost another patient to Cholera and decided to do some further research.
I think cholera could be caused by something else like water
He had his hypothesis.
H2O could be the cause of this all
After a lot of reserch John Snow realised all off the deaths happened near the Broad Street pump .Unknowingly one of the cesspool where leaking into the pump, making the water very dirty. This was why everyone was dying.
Immediately, John Snow told the people about this and they removed the handle of the Broad Street Pump
If not for John Snows determination all of London will be ... you know x x _