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.