If your belly is hanging over your waistband more than it used to, you probably tend to point the finger at all those macho nachos you've been eating. And you should - the macho nachos are definitely to blame. But: if you have any extra large friends, you ought to hold them accountable too. Research suggests obesity is contagious - it can literally spread from person to person, like a fashion trend. And it's not the only social contagion out there - depression and even goodness appear to be communicable too. Harvard medical sociologist Nicholas Christakis studies social contagion, and he speaks with Faith Salie on Fair Game.