I used to work in an office where they got furious with me if I came into work and was sick. "Why are you here?" they'd ask me. "Are you trying to get the rest of us sick?" They'd feel so strongly about it that they would send me home for the day. It makes sense because self-interest the motivator: getting sick is lousy, so better to let me have the time off than risk it.

And yet, we don't act this way about emotions. Moods, emotions, and thoughts spread like diseases among groups of people. General malaise, frustration, or anger are transmitted like contagions through networks of people. Psychologists have found that if you spend time in the company of depressed and pessimistic people who feel like losers, you yourself will start to feel more depressed, pessimistic, and like a loser.

And so, we make ourselves sick. What really amazes me is that we don't send these people home. I wonder why this is, since the damage can be far worse, and more permanent, to those around you if you're going around spreading a negative world view instead of just the common cold.