We've found that certain search terms are good indicators of flu activity. Google Flu Trends uses aggregated Google search data to estimate flu activity in your state up to two weeks faster than traditional flu surveillance systems.Google's data match up well with U.S. Centers for Disease Control and Prevention (CDC) data on flu incidence, but are available 1-2 weeks before CDC data. Here's a comparison of Flu Trends and CDC data for previous years. Plus, Google makes the information easily accessible and user-friendly.
Each week, millions of users around the world search for online health information. As you might expect, there are more flu-related searches during flu season, more allergy-related searches during allergy season, and more sunburn-related searches during the summer.
I think this a fantastic use of the massive amount of raw information on the internet. It's amazing what a person can do with a brain and an internet connection these days.