We've found that certain search terms are good indicators of flu activity. Google Flu Trends uses aggregated Google search data to estimate flu activity in your state up to two weeks faster than traditional flu surveillance systems.So by using the use of search terms as a proxy for people's deepest wishes and concerns, and by extension as a proxy for their physical state, Google is providing a public service - letting us know where and when the flu is spreading. Faster than the Centers for Disease Control itself. Nothing nefarious about that.
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.
During the 2007-2008 flu season, an early version of Google Flu Trends was used to share results each week with the Epidemiology and Prevention Branch of the Influenza Division at CDC. Across each of the nine surveillance regions of the United States, we were able to accurately estimate current flu levels one to two weeks faster than published CDC reports.