Gunther Eysenbach, a health-policy professor with the University of Toronto, says that three years ago, he proposed to Google the idea of analyzing search data in the same way that Flu Trends does. However, he claims that he did not receive a response. Without direct access to Google's data, Eysenbach decided to take out an AdWords advertisement on the site. The ad not only gave him access to search data for the terms "flu" and "flu symptoms," but it also showed the number of people who clicked on the advertisements.

"If you count the number of clicks, then you can get a better prediction," Eysenbach says. "You can weed out the number of people who have flu symptoms from the number of people who just hear about the disease from media reports."

In a paper published in 2006, Eysenbach showed a strong positive link between Internet users' searches and the incidence of influenza in a certain region--in this case, Canada. While Eysenbach's work is briefly mentioned in Google's paper, the researcher says that he was "in a mild state of shock" that Google had decided not to collaborate with him.

In its announcement, Google tried to head off any concerns that the tool would impact users' privacy, stressing that, while the research "aggregates hundreds of billions of individual searches," the information is anonymized and therefore cannot be employed to track an individual user.

MIT's Eagle, however, says that people should expect that their data will increasingly be used in the future. "At the end of the day, this type of data is a fact of life in the 20th century," he says. "We could ignore it and pretend it doesn't exist, or we could use this data, without compromising privacy, in ways that can help people."