Eli recently posted a piece on a study by Nicholas Christakis and James Fowler that looked at the spread of influenza in social networks. I'm currently reading their book, Connected, which explores many issues regarding social networks. It's an interesting read, particularly the chapter on how these networks influence health, including the contagion of obesity and suicide, and the complexity of transmission of sexually transmitted diseases. They also note how social networks could be exploited to improve health. Here's one interesting way: it's possible to achieve the effect of randomly vaccinating 99% of individuals in a population by vaccinating only 30% of the acquaintances of randomly selected individuals. There's also a TED talk on social networks by Christakis here. Lots of food for thought.