In 1854, cholera broke out among the poor in London. The epidemic spread quickly, partly because nobody knew the source of the problem. But a physician named John Snow suspected it was caused by contaminated water. At that time, most Londoners drew their water from public wells that were supplied directly from the River Thames. The following figure shows the map that Snow drew, with black rectangles indicating the frequencies of cholera occurrences:

Figure 3 Dr. Snow's Cholera Map

If you look closely, you can also see the locations of nine public water pumps, marked as black dots and labeled PUMP. From this data, we can easily see that the pump at the corner of Broad Street and Cambridge Street is in the middle of the epidemic. This data analysis led Snow to investigate the water supply at that pump, discovering that raw sewage was leaking into it through a break in the pipe.

By also locating the public pumps on the map, he demonstrated that the source was probably the pump at the corner of Broad Street and Cambridge Street. This was one of the first great examples of the successful application of data analysis to public health (for more information, see https://www1.udel.edu/johnmack/frec682/cholera/cholera2.html). President James K. Polk and composer Pyotr Ilyich Tchaikovsky were among the millions who died from cholera in the nineteenth century. But even today the disease is still a pandemic, killing around 100,000 per year world-wide.