The industrial era of the 19th and early 20th centuries
created cities that provided economic opportunity to millions of
foreign immigrants and migrants from rural and small-town
America. But with so many people moving into cities,
overcrowding, poor sanitation, substandard housing and high
poverty rates contributed to widespread outbreaks of infectious
diseases, such as cholera and tuberculosis. The new professions
of public health and urban planning developed in response to the
desire to improve living conditions in growing urban centers.