One of the key turning points in the public health movement came when London was suffering from deadly outbreaks of cholera. When cholera would hit a neighborhood, people would start dying within 48 hours of when they contracted the disease. But no one knew that the disease was waterbourne and coming from their wells. Dr. Snow was writing papers and using numbers to track the disease and show how it spread, but no one was listening. Then, one day, he used a map to tell the story of one specific neighborhood's outbreak. That map changed London's response to the disease and is still held up today as one of the more imporant maps (especially in lives saved and civilization changed) in the history of the world.
Stephen Johnson, author of Ghost Map, tells the story in just 10 lively minutes as a TED Talk. I've used Dr. Snow's map in my geomatics lectures for years, but have never told the story like this. The talk is especially fun since he tells his yarns of water-bourne intestinal diseases while his audience eats a lunch.