A significant event in the history of epidemiology and public health was Dr. John Snow (John Snow (physician))'s study of an 1854 outbreak (Broad Street cholera outbreak) of cholera in Soho.
He identified the cause of the outbreak as water from the public water pump located at the junction of Broad Street (now Broadwick Street) and Cambridge Street (now Lexington Street), close to the rear wall of what is today the ''John Snow'' public house - wikipedia 
A replica of the pump, with a memorial plaque and without a handle (to signify John Snow's action to halt the outbreak) was erected near the location of the original pump - wikimedia
John Snow memorial, with John Snow pub shown in the background - wikimedia
John Snow mapped the addresses of the sick, and noted that they were mostly people whose nearest access to water was the Broad Street pump. He persuaded the authorities to remove the handle of the pump, thus preventing any more of the infected water from being collected.
The spring below the pump was later found to have been contaminated with sewage. This is an early example of epidemiology, public health medicine and the application of science—the germ theory of disease—in a real-life crisis.
Science writer Steven Johnson (Steven Johnson (author)) describes the 2006 appearance of places related to the Broad Street Pump cholera outbreak:
Almost every structure that stood on Broad Street in the late summer of 1854 has been replaced by something new – thanks in part to the Luftwaffe, and in part to the creative destruction of booming urban real estate markets. (Even the streets' names have been altered. Broad Street was renamed Broadwick in 1936). The pump, of course, is long gone, though a replica with a small plaque stands several blocks from the original site on Broad Street. A block east of where the pump once stood is a sleek glass office building designed by Richard Rogers with exposed piping painted a bold orange; its glassed-in lobby hosts a sleek, perennially crowded sushi restaurant. St. Luke's Church, demolished in 1936, has been replaced by the sixties development Kemp House, whose fourteen stories house a mixed-use blend of offices, flats, and shops. The entrance to the workhouse on Poland Street is now a quotidian urban parking garage, though the workhouse structure is still intact, and visible from Dufours Place, lingering behind the postwar blandness of Broadwick Street like some grand Victorian fossil. (…) On Broad Street itself, only one business has remained constant over the century and half that separates us from those terrible days in September 1854. You can still buy a pint of beer at the pub on the corner of Cambridge Street, not fifteen steps from the site of the pump that once nearly destroyed the neighbourhood. Only the name of the pub is changed. It is now called The John Snow