Research on contagious happiness

I found a few interesting writings on studies done on contagious happiness. These recent scientific research show that the level of happiness can increase within one's social network. Also, how one's happiness level can be closely affected by people surrounding them, even strangers.

British Medical Journal written by James H Fowler (professor at the University of California) and Nicholas Christakis (Harvard professor)

Here's how other publications/individuals have responded
US News and World Report
New York Times: Health: Image: Contagious Happiness
Scientific American Podcast
Merv Benson: Contagious Happiness
Scott Lazerson: Contagious Happiness
Globe and Mail: Happiness is Contagious
Healthy Ontario