Via TED:

We’re all embedded in vast social networks of friends, family, co-workers and more. Nicholas Christakis tracks how a wide variety of traits — from happiness to obesity — can spread from person to person, showing how your location in the network might impact your life in ways you don’t even know.

Related: New mathematical method may help tame big data

Update 20100522: A video at Big Think (The Power of Your Friend’s Friend’s Friend):

Update 20100617: The Social Situation of Breaking Up (The Situationist)

YouTube also has his lecture at Darthmouth College from 2008 on obesity and the social network called: Eat, Drink, and Be Merry: The Spread of Obesity (69m 57s)

Update 20100713: How are sadness and happiness like diseases? They’re infectious, study finds

Update 20100916: New TED talk: How social networks predict epidemics