Google has discovered that they can accurately predict the incidences of flu throughout the United States, simply by tracking flu-related Google searches. It is just as accurate as the method used by the Center for Disease Control, as this graph illustrates:
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 from each state and region are added together. We compared our query counts with data from a surveillance system managed by the U.S. Centers for Disease Control and Prevention (CDC) and found that some 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 various regions of the United States. Our results have been published in Nature.
Because of this, you can actually track where the flu is spreading, by going to Google Flu Trends.