There was a really interesting study by Nicholas Christakis and James Fowler in the May 15th Economist (here) and posted online (full manuscript here). The general thought behind the study was that people in the center of social networks, the ones with more friends or connections, would be more likely to be infected with influenza sooner. Thus, if you could identify people in the center they could serve as an early warning system for flu. The problem is, this would take a significant amount of effort. The insight into this problem comes in the form of what is called the friendship paradox. This 'paradox' suggests that your friends have more friends than you do.
To test this theory, Christakis and Fowler identified 319 students and then 425 of their friends. If friendship paradox would work in identifying influenza then the 425 should get flu earlier. So they followed the 744 students from September to December 2009 during the H1N1 epidemic and found that the friends, the 425 more likely to be in the center of the network, developed influenza (self-diagnosed and confirmed) around 2 weeks earlier. In fact, self-reported symptoms peaked 83 days earlier and visits to healthcare facilities peaked 46 days earlier in the connected group. Maybe this is a new method that could be added to other monitoring systems. Either way, come influenza season, stay away from your friends, especially the ones who are really friendly.