used by CDC. Of course, past performance is no guarantee of future results. Our system is still very experimental, so anything is possible, but we're hoping to see similar correlations in the coming year.

The doctors keep track of the percentage of their patients who have an influenza-like illness, also known as an "ILI percentage". CDC and state health departments collect and aggregate this data each week, providing a good indicator of overall flu activity across the United States.

So why bother with estimates from aggregated search queries? 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. This is an exciting development, because early detection of a disease outbreak can reduce the number of people affected.