Diseases are contagious, and so, it turns out, are behaviors like quitting smoking. Researchers at Harvard and the University of California at San Diego have found that a person in one part of a social network was 20% more likely to quit smoking if a person in another part of that network quit, even if they were several degrees removed from each other, or even remarkably if they didn't know each other at all. The person quitting influences others who eventually influence you.