Dr. Snow knew that he had identified the transmission method for the "cholera poison." This "poison" was later identified as the bacterium VIBRIO CHOLERAE. Cholera leads to an infection of the small intestine which results in extreme diarrhea which may lead to massive dehydration and death. The disease can be treated by giving the victim a lot of fluids -- either by mouth or intravenously (directly into the blood stream).
As sanitation and more effective water treatment increases throughout the world, the number of cholera victims decreases. Cholera has existed in Northern India for centuries and it is from this region that regular outbreaks are spread. The disease diffuses as people travel from the source area. With modern transportation such as airplanes, diseases can be spread a great distance very rapidly.
In the nineteenth century, there were several outbreaks of cholera in London. In the 1849 outbreak, a large proportion of the victims received their water from two water companies. Both of these water companies had the source of their water on the Thames River, just downstream from a sewer outlet. In an 1854 outbreak, most of the deaths occurred within the area of the Southwark and Vauxhall Water Company. Fortunately, just before the outbreak, the Lambeth Water Company relocated their water source to a less polluted point so fewer deaths occurred among their customers. The distribution of deaths was one of the primary factors which proved that the deaths were caused by ingestion.
Dr. Snow plotted the distribution of deaths in London on a map. He determined that an unusually high number of deaths were taking place near a water pump on Broad Street. Snow's findings led him to petition the local authorities to remove the pump's handle. This was done and the number of cholera deaths was dramatically reduced.
The work of Doctor Snow stands out as one of the most famous and earliest cases of geography and maps being utilized to understand the spread of a disease. Today, specially trained medical geographers and medical practitioners routinely use mapping and advanced technology to understand the diffusion and spread of diseases such as AIDS and cancer. A map is not just an effective tool for finding the right place, it can also be a life saver.