"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."
Their search data aligns well to actual medical data and, because of the time taken for CDC data collection/analysis, can be made available 2 weeks ahead of the news from health agencies. I seldom get the flu, but if you do, Google has surely invented something (else) that can help.