If you'd like to read more about John Snow and the London cholera epidemic there is a nice book by Steven Johnson that came out in 2006 called 'The Ghost Map: The Story of London's Most Terrifying Epidemic--and How It Changed Science, Cities, and the Modern World.' As part of the publicity for the book they made an amusingly educational (or educationally amusing) animated video that is well worth 3 minutes of your time.
This graphical approach to depicting to disease has been adopted by a recent website Who is Sick? Here you can find out just what is going around in your community. I'm not sure they've really reached critical mass yet but if a lot of people reported their sickness on the website it would be a very useful tool. Currently you have to look at relatively large urban areas such as LA or the Bay Area to see its potential.