One little bit of information design created in 1854 changed conventional thinking in a way that stopped the London cholera outbreak of 1866, saving hundreds of thousands of lives, and preventing any more outbreaks since.  Dr. John Snow and a minister named Henry Whitehead did a study and mapped their findings to prove Dr. Snow's hypothesis that Cholera was a waterborne disease.  As a result the government changed their policy and their communications to the public, sparing countless hundreds of thousands of lives by stopping the next outbreak and preventing any further outbreaks.

This little piece of information design is Dr. Snow's map indicating the number of deaths in proximity to a particular drinking water well on Broad Street (indicated with an X).  This "ghost map" was a key element, a linchpin, that finally convinced bureaucrats and doctors alike that cholera was spread via drinking water.  This bit of knowledge taught the Western world about the importance of sanitation in cities, which is what allows major cities to even exist at all today.  I learned about it from this great TED Talk by Steven Johnson.  Click here to check it out.  It's about 10 minutes long.