This is so simple, and according to Google, it can predict a flu outbreak 2 weeks sooner than "traditional systems."
When people across the country begin to feel sick, they turn to Google and search for "flu symptoms." Google is able to track these queries geographically, and aggregate the data which they are now sharing on the Google Flu Trends website: http://www.google.org/flutrends/ There's a graph, an interactive map of the USA, a link to the CDC, FAQs, and even a link to download the raw data!
Read more about it in the New York Times: http://www.nytimes.com/2008/11/12/technology/internet/12flu.html
I wonder if a Google Earth kml file is in the works???