Google Flu Trends, which launched in 2008, monitors web searches across the US to find terms associated with flu activity such as “cough” or “fever”. It uses those searches to predict up to nine weeks in advance the number of flu-related doctors’ visits that are likely to be made. The system has consistently overestimated flu-related visits over the past three years, and was especially inaccurate around the peak of flu season – when such data is most useful. In the 2012/2013 season, it predicted twice as many doctors’ visits as the US Centers for Disease Control and Prevention (CDC) eventually recorded. In 2011/2012 it overestimated by more than 50 per cent.
http://www.newscientist.com/article/dn25217-google-flu-trends-gets-it-wrong-three-years-running.html#.UyK6qce7hBo