The "comic book" link on Correlate is awesome. More sites should have documentation like this.
http://www.google.com/trends/correlate/comic
In epidemiology, this is known as syndromic surveillance, and it usually involves checking drugstores for purchases of cold medicines, doctor’s offices for diagnoses, and so forth. But because acquiring timely data can be difficult, syndromic surveillance has always worked better in theory than in practice. By looking at search queries, though, Google researchers were able to analyze data in near real time. Indeed, Flu Trends can point to a potential flu outbreak two weeks faster than the CDC’s conventional methods, with comparable accuracy.
The Correlate comic was drawn by Manu Cornet, who's a Googler: https://plus.google.com/106154493978299996445/posts