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Kai Ryssdal: Some estimates say nearly 100 million Americans are obese. And although it’s not contagious, the New England Journal of Medicine reports today obesity is spreading through social networks — from spouse to spouse, sibling to sibling, and friend to friend. From WBUR in Boston, Curt Nickisch has more.

Curt Nickisch: One of the lead authors of the study is Nicholas Christakis from Harvard.

Nicholas CHRISTAKIS: This result is not about birds of a feather flocking together.

Christakis says obesity spreads. If your close friend becomes obese, you are then almost 60 percent more likely to become obese, too.

CHRISTAKIS: People around you gain weight. Your attitudes about what constitutes an acceptable body size changes, and you might then follow suit and emulate that body size.

The surprising discovery helps explain the costly epidemic. In 2004, medical expenses and lost worker productivity set the U.S. back $93 billion.

Eric Finkelstein helped the government come up with that figure. He’s a health economist at RTI International. Despite the hefty price tag, Finkelstein says programs to get people to lose weight cost more than they’re worth.

Eric FINKELSTEIN: My uncle is a very rich, very intelligent attorney, who happens to be very obese. And it’s just not obvious to me why we would want to spend our hard earned tax dollars to get my uncle to weigh less than he currently does.

But now this new research gives the health care industry a better understanding of obesity, and maybe how to fight its spread — as in collectively, not individually. Harvard’s Christakis says spending a thousand dollars to help someone lose 20 pounds might pay off if the benefit spreads from friend to friend.

CHRISTAKIS: So the $1,000 we have spent, instead of costing $50 per pound lost in the population, it actually only costs $5 per pound lost.

Making such efforts penny-wise, pound-wise after all.

In Boston, I’m Curt Nickisch, for Marketplace.