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Toronto – Obesity can spread from one person to another like the flu or a fad, researchers report today in a first-of-its-kind study that helps explain – and could help fight – one of the nation’s biggest public health problems.

And how did they reach this conclusion?

The researchers found that when one spouse became obese, the other was 37 percent more likely to do so in the next two to four years, compared to other couples. If a man became obese, his brother’s risk rose by 40 percent.

The risk rose even more sharply among friends – between 57 and 171 percent, depending on whether they considered each other mutual friends. Moreover, friends affected friends’ risk even when they lived far apart, the researchers found.

And why did they reach this conclusion?

“It’s almost a cliche to speak of the obesity epidemic as being an epidemic. But we wanted to see if it really did spread from person to person like a fashion or a germ,” said Nicholas Christakis of Harvard Medical School, who led the study published today in the New England Journal of Medicine. “And the answer is, ‘Yes, it does.’ We are finding evidence for a kind of social contagion.”

Watch out. They’ll be quarantining fat people next. Or banning them from friendships:

“What spreads is an idea. As people around you gain weight, your attitudes about what constitutes an acceptable body size changes, and you might follow suit and emulate that body size,” Christakis said.

Adds the New York Times:

It may also mean that the way to avoid becoming fat is to avoid having fat friends.

That is not the message they mean to convey, say the study investigators, Dr. Christakis and his colleague, James H. Fowler, an associate professor of political science at the University of California, San Diego.

You do not want to lose a friend who becomes obese, Dr. Christakis said. Friends are good for your overall health, he explained. So why not make friends with a thin person, he suggested, and let the thin person’s behavior influence you and your obese friend?

Speaking of spreading ideas, expect the idea that fat people are pariahs to spread, too:

Other researchers used words like “brilliant” and “groundbreaking” to describe the work and said it is likely to lead to a flurry of new research.

“This is one of the most exciting studies in medical sociology that I’ve seen in decades,” said Richard Suzman, director of the behavioral and social research program at the National Institute on Aging, which funded the study. “I think these results are going to shift the way we think about some of these supposedly non-communicable diseases.”