His work shows how phenomena as diverse as obesity, smoking, emotions, ideas, germs, and altruism can spread through our social ties, and how genes can partially underlie our creation of social ties to begin with. His work also sheds light on how we might take advantage of an understanding of social networks to make the world a better place.
Christakis explains that he and James Fowler (co-author of Connected) have become obsessed with trying to figure out how and why we form networks and how and why they affect us.
Once you start mapping these networks -- they’re so intricate and so beautiful and so interesting -- you just can’t help but wonder why we humans make them. Why does a spider weave its web? Why does the web have a particular kind of shape? It’s not a coincidence. You look at these webs and you think, “My God, what purpose do they serve? And, how do they affect us?”
Watch his TedTalk presentation, "The Hidden Influence of Social Networks".
Here is the web site for their book Connected. And here is a link to another interview.