From the Google Flu Trends website:
Each week, millions of users around the world search for health information online. 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. You can explore all of these phenomena using Google Insights for Search. But can search query trends provide the basis for an accurate, reliable model of real-world phenomena?
We have found a close relationship between how many people search for flu-related topics and how many people actually have flu symptoms. Of course, not every person who searches for “flu” is actually sick, but a pattern emerges when all the flu-related search queries are added together. We compared our query counts with traditional flu surveillance systems and found that many 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 different countries and regions around the world. Our results have been published in the journal Nature.
[…] So why bother with estimates from aggregated search queries? Traditional flu surveillance is very important, but most health agencies focus on a single country or region and only update their estimates once per week. Google Flu Trends is currently available for a number of countries around the world and is updated every day, providing a complement to these existing systems.
For epidemiologists, this is an exciting development, because early detection of a disease outbreak can reduce the number of people affected. If a new strain of influenza virus emerges under certain conditions, a pandemic could ensue with the potential to cause millions of deaths (as happened, for example, in 1918). Our up-to-date influenza estimates may enable public health officials and health professionals to better respond to seasonal epidemics and pandemics.
Experimental Flu Trends
For countries with Experimental Flu Trends, we found aggregated flu-related queries which produced a seasonal curve that suggested actual flu activity. However, these estimates have not been compared with official influenza surveillance data. The experimental estimates are available for download from each Experimental Flu Trends country page. If you are a public health official with ILI data available for one of the experimental countries, please contact us and we will work to validate our estimates against the historic data.Protecting User Privacy
At Google, we are keenly aware of the trust our users place in us, and of our responsibility to protect their privacy. Google Flu Trends can never be used to identify individual users because we rely on anonymized, aggregated counts of how often certain search queries occur each week. We rely on millions of search queries issued to Google over time, and the patterns we observe in the data are only meaningful across large populations of Google search users. You can learn more about how this data is used and how Google protects users’ privacy at our Privacy Center.