Urbanization, and the "plague among children" that emerged from nothingness
I had never heard of diphtheria until moving to the US, but then I lived in fairly spread out areas in Australia for my first 19 years. That is none of them had large populations. Also, there seemed to be no knowledge about it.
However, I just asked a person from Hong Kong if they had heard of it (or the Chinese name for it) and she had not. Also, a quick search revealed no info on the history of diphtheria in China, which is strange because they have had very large cities for quite a while and HK has been a large city for quite some time. Maybe opium was a bigger problem for them.
Trying to reconcile this account of diphtheria with, say, Laura Ingalls Wilder's account of it, where her whole family came down with it in their rural home in DeSmet ND in 1888. Surely that wasn't on account of urbanization? Since the contagion seemed to be both from "walking" cases as well as sewage... would speedier transport (i.e. railroads) have extended the reach of the disease into rural communities connected by train? I wonder if there's any way to compare communities with regular train service, to communities that were a good distance from the nearest railway?
Lots of city electric wires installed in the cities around that time would be my guess. Pictures I have seen of the wires look ridiculous in density and very low to the ground.