The work of John Snow is instrumental to the field of epidemiology. In the mid-19th century, during what was the third major outbreak of cholera, he created the following map showing the clusters of cholera cases in London's Soho neighborhood. Stacked rectangles were used to indicate the number of cholera cases in a particular location. This was a major breakthrough for the fight against cholera because, at the time, it wasn't clear what was causing it. According to Wikipedia, there were two main competing theories. There was the miasma theory, which posited that cholera was caused by bad particles in the air. And there was the germ theory, which posited that cholera could be passed along through food and/or water.