"Google sees a potential profit center from targeting its vast user base with advertising that is related to health issues," Chester said. The company's announcement of Flu Trends in fact shows to pharmaceutical and medical markets precisely the kind of sophisticated analysis the company can do with search data to enable highly targeted medical marketing, he said. "This is about taking the tracking data that Google has at its disposal and focusing it on generating a new profit center for the company," Chester said.

Pam Dixon, executive director of the World Privacy Forum, echoed similar concerns and questioned whether the anonymization techniques used by Google provided enough of a guarantee that a search term could not be traced back to specific individuals. She pointed to an incident two years ago where on a public Web site. The search queries had supposedly been anonymized by AOL, but it was still relatively easy to track specific search terms back to IP addresses and even individuals in many cases, Dixon said.

Mike Yang, senior product counsel at Google, downplayed privacy concerns related to Flu Trends and insisted that the tool uses no personally identifiable data.

"Flu Trends uses aggregated data from hundreds of millions of searches over time," Yang said today in an e-mail. "Flu Trends uses aggregations of search query data which contain no information that can identify users personally. We also never reveal how many users are searching for particular queries."

Yang noted that the data used in Flu Trends comes from Google's standard search logs. He also referenced , authored by the Google Flu Trends team, which he said explains the methodology behind the tool.