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

The study, involving more than 12,000 people tracked over 32 years, found that “social networks” play a surprisingly powerful role in determining an individual’s chances of gaining weight, transmitting an increased risk of becoming obese from wives to husbands, from brothers to brothers, and from friends to friends.

The researchers found that when one spouse became obese, the other was 37% 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%.

The risk rose even more sharply among friends — between 57% and 171%, depending on whether they considered each other mutual friends. Moreover, friends affected friends’ risk even when they lived far apart, and the influence cascaded through three degrees of separation before petering out, the researchers found.

“It’s almost a cliché 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 being published tomorrow in the New England Journal of Medicine. “And the answer is, ‘Yes, it does.’ We are finding evidence for a kind of social contagion.”

Mr. Christakis stressed that the researchers are not saying that obesity is literally caused by a virus or some other pathogen, or that factors such as poor diet, lack of exercise, or a biological propensity are unimportant. Rather, the findings suggest that once a person becomes obese for whatever reason, it may make it more socially acceptable for people close to him or her to gain weight, and that new social norms can proliferate quickly.

“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,” Mr. Christakis said. “It may cross some kind of threshold, and you can see an epidemic take off. Once it starts, it’s hard to stop it. It can spread like wildfire.”

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 noncommunicable diseases.”

In addition to offering novel insights into the obesity epidemic, the discovery could suggest new tactics for stemming the seemingly inexorable trend. The findings lend support to treating people in groups or even whole communities, for example. The researchers noted that their study also showed that people who were close to someone who lost weight were more likely to get thinner.

“If these close social environments can promote a disease, they can also promote solutions to disease,” said William Dietz of the federal Centers for Disease Control and Prevention. “These same social networks might be used to turn a disease like obesity around.”

The proportion of obese Americans has been rising steadily for decades, and more than two-thirds of American adults are now overweight, including one-third who are obese. Obesity boosts the risk for a host of health problems, including diabetes, heart disease, and cancer.