A study out today shows that obesity is contagious. If your best friend gets fat, your chances of gaining weight more than double.
Malcolm Gladwell fans will recall his reporting that suicide among teenagers can be contagious as well.
So is terrorism, of course. And spamming. And graffiti. [And becoming a millionaire, getting your company funded, not dropping out of high school and learning how to shop for bargains, too.]
The most important thing you can do is choose who you're hanging out with. The second high-leverage thing is to put dynamics in place that reinforce the ideas you'd like to see spread. Celebrate the heroes. Make it easy for those ideas to spread...