Using statistics gathered from searches entered into its search engine, Google is exploring its ability to predict outbreaks of the flu. In other words, Google is measuring the trend, so to speak, to use the Web to seek information or guidance regarding health issues—perhaps by searches that query ‘flu symptoms’ or similar. For some additional information, see the Guardian article.
Of course, the Centers for Disease Control and Prevention also track the flu virus. See the Flu Activity and Surveillance page.