John Snow is the father of modern epidemiology, having tracked the source of a cholera outbreak in London in 1855, thanks to some good old fashioned detective work. It’s a story worth reading, but I won’t go into it here.
At any rate, the UCLA Department of Epidemiology has an interactive online map of London in 1859. You can zoom in on the map down to an amazing level.
I actually have a copy of the map (framed for posterity) that I acquired when I was in my victorian role playing phase. It was hard to find then. Fortunately, you don’t have to conduct the smae long search that I did. You can just look at it online.