Even if you're not aware of it, it's likely that your emotions will influence someone around you today.

This can happen during our most basic exchanges, say on your commute to work. "If someone smiles at you, you smile back at them," says sociologist Nicholas Christakis of Yale University. "That's a very fleeting contagion of emotion from one person to another."

But it doesn't stop there. Emotions can spread through social networks almost like the flu or a cold. And the extent to which emotions can cascade is eye-opening.