Dec. 4, 2008 -- Could happiness be contagious?

New research from Harvard Medical School and the University of California, San Diego suggests that happiness is influenced not only by the people you know, but by the people they know.

The study showed that happiness spreads through social networks, sort of like a virus, meaning that your happiness could influence the happiness of someone you've never even met.

Sadness spreads too, but much less efficiently, says study co-author James H. Fowler, PhD, of the University of California-San Diego.

"We have known for a long time that there is a direct relationship between one person's happiness and another's," Fowler tells WebMD.

"But this study shows that indirect relationships also affect happiness. We found a statistical relationship not just between your happiness and your friends' happiness, but between your happiness and your friends' friends' friends' happiness."