The most important factors in improving the health of the people of the United States are clean water, clean air and control of sewage. Cleaning the environment has prevented and cured more illnesses than any discoveries in medicine or any new technology. In the 19th century, slum sewage mixing with well water caused cholera epidemics. In addition, congested neighborhoods aided the spread of tuberculosis throughout urban America.
No one understood "germ theory" or recognized that specific bacteria caused particular diseases until the late 19th century, but scientists and the public health establishment did start to see a link between sickness and refuse. For instance, in New York City in 1835, more than 10,000 horses dropped about a half million pounds of manure a day on the streets of the city. This, along with human waste, quickly contaminated the local wells of Manhattan. Approximately 3,500 New Yorkers died in the resulting cholera epidemic.
By the middle of the 19th century, after a series of epidemics, New York, like other cities, built reservoirs and sewers, improved street cleaning and passed housing reform legislation. These efforts greatly reduced death and illness rates by the turn of the century.
In the early 20th century, progressive-era reform continued to improve the health of Americans, through the inspection of milk, the creation of visiting nurse services, and the discovery of vaccines for diseases like polio and measles, as well as mechanisms to ensure universal vaccination.