Now, Google itself is getting in on the forecasting game. The company said it will scrutinize people's searches to provide what it calls "an early-warning system" for flu outbreaks.

The company cross-referenced queries dating back to 2003 with data from the Center for Disease Control and discovered that searches for flu-related keywords correlated with outbreaks. "If we tally each day's flu-related search queries, we can estimate how many people have a flu-like illness," Google wrote in a blog post about the initiative.

The new flu tracker offers information about flu outbreak by state. While Google didn't specify what information it looked at, the company apparently examined users' IP addresses to determine which state their searches originated from.

It's hard to find fault with the flu-tracking plan, which the company clearly thinks will benefit public health. Still, this program highlights the fact that Google has vast amount of data about Web users at its disposal -- which has long unnerved privacy advocates.

Google acknowledges the potential privacy issues, but says that all search queries are aggregated and "anonymized." "Flu Trends can never be used to identify individual users because we rely on anonymized, aggregated counts of how often certain search queries occur each week."

Whether that's enough to quell concerns remains to be seen. But some commentators have already condemned the proposal. "How comfortable do we all feel about our supposedly privacy-protected searches being used for ends that might be defined as in the public interest," Chris Matyszczyk asks.

This isn't Google's first venture into public health. In February, the company began working with the Cleveland Clinic to allow 10,000 patients to store their medical information online.

That project also drew the ire of some privacy advocates, with the World Privacy Forum warning that federal law limits the release of patient information by doctors and hospitals, but doesn't necessarily protect information people have placed online with companies like Google.

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