It's a Monday at the office, and you're feeling kind of blah. Then, your always-chipper coworker comes over, compliments your haircut, shows you a picture of her adorable baby son and recommends a great restaurant. After she leaves, you can't wipe that stupid grin off your face. You even start whistling as you put the covers on your TPS reports.

Well there's a scientific reason for that, my friend -- cheer is contagious. Scientists from Harvard and UC San Diego found that knowing someone who is happy makes you 15.3 percent more likely to be happy yourself.

And you don't even need to know the person; experts say the bliss bug extends up to three degrees of separation, like a friend of your sister's friend. But proximity can matter -- a happy friend who lives within a half-mile makes you 42 percent more likely to start your day smiling, but if she moves two miles away, your delight drops by half.

The researchers gleaned this information from studying 5,000 people over the course of 20 years. They think spreading satisfaction is evolutionarily useful because it makes groups run more smoothly (you help more people and give more of your time and money, for instance). It's also been reported that companies with happy employees make more money. Take that, Bill Lumbergh.

Joy also journeys through social networks. Why? Because "your emotional state depends not just on actions and choices that you make, but also on actions and choices of other people, many of which you don't even know," said Dr. Nicholas A. Christakis, a physician and medical sociologist at Harvard who co-wrote the study. Felicity doesn't flow as freely at work, however, because you're more likely to encounter someone who stops your contentment from circulating.

Like mirth, misery can also multiply. But don't ditch your dismal friends just yet. You can build up mental defenses to ward off their virus.

Tell us: Have you ever caught the happy bug from a friend or coworker?