They might not know who you are. But they can make you fat or thin, they can make you smoke or quit, they can make you happy or sad—and they don’t even mean to. They do know the people that you know—and that’s how your network of friends, their friends, and their friends’ friends influence you. And rest assured, you’re doing the same thing to them.

This process, called social contagion, was made famous in the widely discussed book Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives (Little, Brown and Co., 2009) by medical doctor and Harvard professor Dr. Nicholas Christakis, Ph.D., and James Fowler, Ph.D., a professor at the University of California, San Diego. Social contagion works like any other kind of contagion—through transmission from one person to another. Instead of germs, however, social contagion transmits behaviors, norms, and emotions.

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