FYI, I came across an article in New Scientist, part of which...

FYI, I came across an article in New Scientist, part of which follows:

....
Launched in 2008, Google Flu Trends 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. David Lazer and his team at Northeastern University in Boston found that the system had been consistently overestimating flu-related visits since 2011, and was particularly inaccurate at the peak of flu season – when such data is most useful.

In the 2012/2013 season, Google Flu Trends predicted twice as many doctors' visits as were actually recorded by the Centers for Disease Control and Prevention (CDC). In 2011/2012 it overestimated by more than 50 per cent (Science, doi.org/rwx).

Lazer says the fixes for Google's problems are relatively simple – much like recalibrating weighing scales. "It's a bit of a puzzle, because it really wouldn't have taken that much work to substantially improve the performance of Google Flu Trends," he says. Merely projecting current CDC data three weeks into the future yields more accurate results than those compiled by Google Flu Trends.
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see http://www.newscientist.com/article/mg22129614.800-google-flu-trends-gets-it-wrong-three-years-running.html (you probably need a subscription to see the whole document).