Google has discovered a close relationship between how many people search for flu-related topics online, and how many people actually have flu symptoms, by comparing their query data with data from the surveillance system managed by the U.S. Centers for Disease Control and Prevention (CDC). Of course, not every person who searches for "flu" is actually sick, but a pattern emerges when all the flu-related search queries from each state and region are added together.

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, and geographically located, very quickly. By releasing, and then visualizing [google.org] the aggregated search data to the public, the resulting influenza estimates may enable public health officials and health professionals to better respond to seasonal epidemics and pandemics.

See also Who is Sick - Sickness Map. Via kottke.org and Flowing Data.