Google Flu Trends aggregates search data to track the spread of infection "up to two weeks faster than traditional systems." Time explains:

The reasoning is that if people are searching for information on the flu, they're probably sick themselves or know someone who is and a geographic cluster of like-minded Googlers could represent a burgeoning outbreak or, worse, the roots of a new pandemic. ... [The] benefit is that they rely not on hospital data but real-time information from people who are in the process of getting sick. "What we are seeing are trends of what people are thinking about at home, perhaps before they might go to see a doctor," says Jeremy Ginsberg, lead engineer of Google Flu Trends.

But Alexis Madrigal shows how they missed the swine flu.

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