Check this out! Google has been using search results to measure the severity of flu outbreaks around the United States.
Google describes the phenomonon as follows.
We have found a close relationship between how many people search for flu-related topics and how many people actually have flu symptoms… We compared our query counts with data from a surveillance system managed by the U.S. Centers for Disease Control and Prevention (CDC) and discovered that some search queries tend to be popular exactly when flu season is happening. By counting how often we see these search queries, we can estimate how much flu is circulating in various regions of the United States.
They go on to say
CDC uses a variety of methods to track influenza across the United States each year. One method relies on a network of more than 1500 doctors who see 16 million patients each year. The doctors keep track of the percentage of their patients who have an influenza-like illness… CDC and state health departments collect and aggregate this data each week, providing a good indicator of overall flu activity across the United States…. It turns out that traditional flu surveillance systems take 1-2 weeks to collect and release surveillance data, but Google search queries can be automatically counted very quickly. By making our flu estimates available each day, Google Flu Trends may provide an early-warning system for outbreaks of influenza.
They’ve got a great video showing their amazingly accurate real-time description flu activity in 2007-8 that came out two weeks ahead of the CDC reports.
Update: looks like the New York Times has an article about this.