The Broad Street cholera outbreak was a severe outbreak of cholera that occurred in 1854 near Broad Street in the Soho district of London, England. This outbreak, which killed 616 people, is best known for the physician John Snow's study of its causes and his hypothesis that contaminated water, not air, was the source of cholera. This discovery came to influence public health and the construction of improved sanitation facilities beginning in the mid-19th century. Later, the term "focus of infection" would be used to describe sites, such as the Broad Street pump, in which conditions are good for transmission of an infection. John Snow's endeavor to find the cause of the transmission of cholera caused him to unknowingly create a double-blind experiment.
In the mid-19th century, the Soho district of London had a serious problem with filth due to the large influx of people and a lack of proper sanitary services: the London sewer system had not reached Soho. Cowsheds, slaughter houses, and grease-boiling dens lined the streets and contributed animal droppings, rotting fluids and other contaminants to the primitive Soho sewer system. Many cellars had cesspools underneath their floorboards, which formed from the sewers and filth seeping in from the outside. Since the cesspools were overrunning, the London government decided to dump the waste into the River Thames, contaminating the water supply. London had already suffered from a "series of debilitating cholera outbreaks". These included outbreaks in 1832 and 1849 which killed a total of 14,137 people.
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