I was a member of a health planning research project in the early 1970s. A WHO expert from Geneva at that time told me that for centuries after the fall of Rome, the largest cities were limited to about half a million inhabitants. At that size a city in the dark ages had a death rate that was so high that immigration and the birth rate could not be its equal, and the city would stop growing.
I don't know why Rome was so much larger at its peak that it was later or than were other European cities at a later date. There were possibly many reasons. The Romans were great engineers and had aqueducts that provided abundant water as well as sewerage to remove human wastes, leading to better hygiene than was possible in later centuries; perhaps the inhabitants of Rome were less often sick from water borne or water washed diseases. The Romans imported food in large amounts and distributed food to poor citizens, so perhaps the population was better nourished. The Romans brought slaves into Rome, and perhaps they brought in slaves faster than the slaves left and died.
In those days as a health planner I helped a small group of students do a senior project studying several alternative locations for a hospital to be built in Cali, Colombia -- the city in which we lived and the health planning project was located. The students developed a model based on the number of patients seeking cared for at each of the existing hospitals; the model was then used to predict how many would go to a new hospital were it located at each alternative place and what that flow would mean in terms of demand at the existing hospitals. (The study proved to be useful and used, and all the students involved were hired by the offices of the Ministry of Health in the Department of Valle.)
Cali, like many cities in developing countries, grew fast and had very different neighborhoods. I lived in one that had all the amenities of a U.S. or European city. There were neighborhoods that had minimal infrastructure, shacks, and high population density. (I remember someone calling those neighborhoods full of soil scrapers, making the word play on sky scrapers -- they had the population per acre you might expect in a neighborhood of sky scrapers, but the shacks barely rose above the soil on which they stood.)
It is too bad we didn't develop a model that would predict health status by neighborhood, leading to applications for prediction of demand on health service facilities (were they to be built), as well as an aid for the planning of infrastructure investment.
The epidemic of Ebola in West Africa suggests how useful a model might be that used housing quality, population density, a the availability of piped water and waste disposal facilities to enable quick prediction of the course of an epidemic and "hot spots".