Signorini, A., Segre, A., and Polgreen, P. (2011). The Use of Twitter to Track Levels of Disease Activity and Public Concern in the U.S. during the Influenza A H1N1 Pandemic. PLoS One.
The authors used data derived from Twitter to track two things: public sentiment specific to H1N1 (swine flu) and actual disease activity. They found that their estimates of influenza-like illness matched reported disease levels.