How did London's sewers defeat cholera for the wrong reason?

By building sewers to get rid of a smell, and removing the real cause of the disease along with it. After the Great Stink of 1858, Parliament funded the engineer Joseph Bazalgette's plan to rebuild London's sewers, expecting enclosed sewers to eliminate the "miasma", the foul air then thought to cause cholera. That premise was wrong. But by carrying sewage away in enclosed sewers, the system removed the cholera bacterium from the water supply, eliminating cholera in the areas it served and reducing typhus and typhoid too.

The real cause had already been spotted. In 1854, a cholera outbreak near Broad Street in London's Soho killed 420 people. The physician John Snow argued that contaminated water, not miasma, was to blame, and the pattern of deaths persuaded parish authorities to remove the handle of the Broad Street pump. The well, it was later found, had been dug three feet from a leaking cesspit. But disabling the pump hardly dented the citywide epidemic, which killed nearly 3,000 people, and once the outbreak subsided officials put the handle back and rejected Snow's theory: accepting it meant accepting the oral-faecal spread of disease, too unpleasant for most of the public to contemplate.

So the change came from the smell. Bazalgette built 82 miles of enclosed underground brick main sewers to intercept sewage, plus 1,100 miles of street sewers, and his innovative use of Portland cement kept the tunnels in good order 150 years later. The Prince of Wales opened the system in 1865, though the whole project took another ten years.