19th century London was literally plagued by cholera until, after a 30-year struggle, public health officials discovered that cholera was water-, not air-borne. The story is told in Steven Johnson's The Ghost Map: The Story of London's Most Terrifying Epidemic - and How It Changed Science, Cities, and the Modern World:

“Thousands of Londoners died while doctors and public-health officials stubbornly clung to the view that the plague was an airborne miasma that hung in the foul atmosphere of the slums and was inhaled by the wretched creatures who lived there…… The researchers mistook correlation for causation: because the stench of the slums was so utterly disgusting and because smell acts so powerfully on our imaginations… The mega-city is in fact something like a natural organism, which begins to fail when one link in the chain breaks. In the case of London's cholera epidemic, the breakdown occurred when a certain class of cleaners, the sewage removers known as night-soil men, found that they could charge as much as one shilling for emptying each cesspit. The poor and their more unscrupulous landlords left the sewage to overflow into the cellars -- and ultimately to leak into the Broad Street well…… Every kind of cure was proposed: opium, linseed oil and hot compresses, smoke, castor oil, brandy -- everything but the simple, obvious remedy of rehydration, which reduces the otherwise fatal disease to a bad case of diarrhea.”

The last great cholera epidemic hit New York in the closing years of WWI. But the ability to subsequently contain it owed a great deal to the Victorian reformers who made great, dense cities possible.