The explosive growth of large urban areas allowed previously localized diseases to spread quickly and infect larger populations.
The increased demand for cheap housing by factory workers led to poorly built homes that inadequately provided for personal hygiene.
These conditions were ideal for the spread of bacteria and infectious disease.
Without organized sanitation systems, bacteria easily passed from person to person through the water and sewage.
As American cities industrialized throughout the nineteenth century, infectious diseases emerged as a real threat.