The Rules of Contagion

Snow’s updated cholera map of Soho

Credit: John Snow Archive & Research Companion. The mark on the

right-hand side is a tear in the original page

Yet on its own, the map was not clear evidence that the water was
responsible. If the cholera outbreak had been the result of bad air
around Broad Street, the pattern would have looked much the same.
So Snow produced a second map, with a crucial addition. As well as
plotting the cases, he worked out how long it would take to walk to
different pumps, drawing a line to show the places for which the
Broad Street pump was nearest. It illustrated the areas that would be