"The best way to cheer yourself is to try to cheer someone else up," wrote Mark Twain in 1896. And 117 years later the human genome has confirmed Mr. Twain's simple formula for true happiness.

We've known for a couple of decades that happiness equals longevity, and unhappiness equals disease and earlier death. Watch a funny video and stress hormone levels plummet 70 percent, while endorphins (feel-good hormones) rise 27 percent. And your immune system gets a boost too.

But no one ever thought to ask: Is all happiness the same? And does it always make you healthier? That is, until now. Recent discoveries have shown your genes get turned on or off (that's called gene expression) by what you and your outside world present to them.

And a careful study of folks who say they are happy reveals that different types of happiness have different effects on gene expression. If you gain happiness from helping other people, you produce gene expressions that make your body more resistant to disease.

But folks who are happy because they're pleasing themselves (how's that reflection lookin', Narcissus?) throw genetic switches that make them less healthy. They have high inflammation, which is not good for the heart or brain, and low antiviral and antibody gene expression. Ah-choo!

So, for genes that smile (love those gene expressions!) and an upbeat outlook on life, volunteer to help a neighbor or with any local organization that inspires you. When you care about other folks and your community, your body will take care of you.