English Physician John Snow (1813-1858) is credited with being the father of epidemiology after his work in tracing a cholera outbreak in London's Soho in 1854. He also studied, and was an early proponent of, anesthesia in medicine. He had had experience of cholera during his medical apprenticeship in Yorkshire but it was when he had a practice in Soho that he tried to find out what spread the deadly disease. The consensus of the cause before then was the 'miasma' theory of noxious bad air. Snow was sceptical of this and set out to try and understand it better.
He talked to the families of the sufferers and discovered that almost all of the victims had drunk water from the public Broad St pump. He proposed removing the handle and incidents of the disease quickly stopped (though Snow himself admitted the outbreak was waning before then). Some years later it was discovered the contents of a cesspit dug very close to the well were contaminating the water.
Snow continued his work, using statistics and a famous dot map showing the cases scattered around Broad St. It took a few years before the water-borne nature of cholera was fully recognised as a major public health issue, but his work led to massive improvements in sewerage and environmental hygiene in cities around the world, saving countless lives in the process.