There's a great article by Clive Thompson in today's New York Times Magazine on so-called "social-contagion effects" - the fact that obesity, smoking, but also happiness seems to spread virally along social connections. Some excerpts:

good behaviors — like quitting smoking or staying slender or being happy — pass from friend to friend almost as if they were contagious viruses.

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And the social effect appeared to be quite powerful. When a Framingham resident became obese, his or her friends were 57 percent more likely to become obese, too. Even more astonishing to Christakis and Fowler was the fact that the effect didn’t stop there. In fact, it appeared to skip links. A Framingham resident was roughly 20 percent more likely to become obese if the friend of a friend became obese — even if the connecting friend didn’t put on a single pound. Indeed, a person’s risk of obesity went up about 10 percent even if a friend of a friend of a friend gained weight.

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Christakis and Fowler hypothesize that these behaviors spread partly through the subconscious social signals that we pick up from those around us, which serve as cues to what is considered normal behavior. Scientists have been documenting this phenomenon; for example, experiments have shown that if a person is seated next to someone who’s eating more, he will eat more, too, unwittingly calibrating his sense of what constitutes a normal meal.

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The subconscious nature of emotional mirroring might explain one of the more curious findings in their research: If you want to be happy, what’s most important is to have lots of friends. Historically, we have often thought that having a small cluster of tight, long-term friends is crucial to being happy. But Christakis and Fowler found that the happiest people in Framingham were those who had the most connections, even if the relationships weren’t necessarily deep ones.

The reason these people were the happiest, the duo theorize, is that happiness doesn’t come only from having deep, heart-to-heart talks. It also comes from having daily exposure to many small moments of contagious happiness.

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In essence, Christakis and Fowler’s work suggests a new way to think about public health. If they’re right, public-health initiatives that merely address the affected individuals are doomed to failure. To really grapple with bad behaviors that spread, you have to simultaneously focus on individuals who are so distant they don’t even realize they’re affecting one another.

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other scientists have used Christakis and Fowler’s work to inspire more potentially practical public-health projects, some of which are now being implemented. Nathan Cobb, a smoking-cessation expert and researcher at the Schroeder Institute for Tobacco Research and Policy Studies, is designing an application that Facebook users can install on their pages when they’re trying to quit smoking. The application will publicly display how long they’ve gone without cigarettes, whether they are using a nicotine patch and how much money they have saved by not smoking. The idea, Cobb says, is to take your invisible, internal battle to quit smoking and make it visible so that it can influence your friends (and friends of friends) who are still puffing away.