Each week, millions of users around the world search for health information online. Google has 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. Google compared 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 Google sees these search queries, they can estimate how much flu is circulating in different countries and regions around the world.
To learn more watch this Google Tech Talk presentation given by Corrie Conrad on Google Flu Trends: