Google has partnered with the CDC to produce Google Flu Trends. Until now, flu activity nationwide has been tracked primarily through CDC surveillance of visits to doctors for influenza-like illness (ILI). Those data are sent to the CDC for analysis on a weekly basis by "sentinel providers", physicians who volunatarily report on how many patients they have seen that week with ILI. Unfortunately, because of the time involved with data collection and analysis, there is generally a one to two week lag in the availability of the information to the public.
The people at Google, working with CDC, came up with the brilliant idea to track the number of online queries about flu in each state or region, on the theory that as more people experienced ILI or were diagnosed with flu, there would be more curious people with flu going to the internet for more information. The advantage of this system is that the information can be tallied daily, providing a completley up-to-date snapshot. For more about Google Flu Trends, click here.