Google Flu Trends

This graph shows historical query-based flu estimates for different countries and regions compared against official influenza surveillance data.

United States Flu Activity
Influenza estimate	Google Flu Trends estimate  United States data

United States: Influenza-like illness (ILI) data provided publicly by the U.S. Centers for Disease Control.

From http://www.google.org/flutrends/about/how.html

Approximately how many cases of flu were there in 2005?

When were Google estimates of flu numbers closest to reported data?

When were Google estimates least accurate?

If you have time, http://www.google.org/flutrends/us/ is interesting as well.