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When, a few years back, scientists and public health officials began using the expression "obesity epidemic," it was controversial.

The term epidemic was traditionally used to describe the rapid spread of infectious diseases such as influenza or polio. But the argument was made that the term could be used in a non-biological sense to refer to widespread and growing social problems.

Now comes word that obesity actually is contagious. Fat is not caused by a virus, bacterium or other pathogen. Rather, it spreads to the waists, thighs and other jiggly regions of the body through social networks.

In a fascinating article in today's New England Journal of Medicine, researchers at Harvard University and the University of California, San Diego, show that if a close friend becomes obese, your likelihood of becoming obese soars by 171 per cent. If it's a casual friend or acquaintance, the risk is still a notable 57 per cent, and your friend's friends have a similarly increased risk of obesity.

By comparison, having an obese sibling increases your risk 40 per cent, and an obese spouse ups your risk 37 per cent. All these risk calculations are done over a long period, 32 years.

The catchy, headline-grabbing interpretation of this study is: Your friends make you fat.

But there is a less sensational and more important point that should be retained.

"These findings reinforce the idea that obesity is not just an individual problem, but a collective problem," Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston, told a media teleconference.

It's not that people who are overweight or obese seek each other out, but rather that friends are growing fatter together.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger, since those around them are bigger, and this sensibility spreads," Dr. Christakis said.

The results of this type of thinking are visible in the streets and striking in the statistics.

In the United States, 66 per cent of adults are now overweight, including 33 per cent who are obese (meaning more than 30 per cent of their body weight consists of fat.)

Americans aren't the only ones losing the battle of the bulge. In Canada, 59 per cent of adults are overweight, including 23 per cent who are obese.

In other words, normal weight is not normal any more.

And what the new study tells us is that, no matter how unhealthy being obese can be (excess weight is a risk factor for diabetes, heart disease, stroke and some forms of cancer), we are able to rationalize excess poundage because it is the norm in our social circle.

Physiologically, gaining weight is easy to explain: If you ingest more calories than you burn, you gain weight. Do so routinely over a period of years and, like the majority of Canadians, you find yourself overweight.

But understanding why and what people eat - quantity and quality - and how active they are is far more complex.

Genetics come into play, but our genes alone do not explain why we need ever-larger jeans.

Far too often in dealing with health issues, particularly public health issues, we ignore or fail to recognize the importance of social factors.

This new study looked at the influence of friendship and the social environments we create for ourselves.

From a public health perspective, the information is valuable: It holds the promise that, if you get one person to deal with their weight, it will have a ripple effect on their social network of friends and family.

Of course, we know this intuitively. Anyone who has ever tried to undertake a diet or exercise regime knows that, without the support of friends and family, it is impossible to maintain the lifestyle change. But the research demonstrates just how powerful these influences can be.

Bear in mind, however, that the new study is derived from the famed Framingham Heart Study, a research project that has been tracking the health of residents of a Massachusetts town since 1948.

The study has provided a gold mine of information, but its one weakness is the lack of racial and socio-economic diversity of its participants.

Friendship no doubt has an important influence on a person's weight, but it probably pales in comparison to matters over which individuals have far less control: income, housing, education, job security, food security and a sense of belonging.

It is no coincidence that rates of overweight and obesity are significantly higher among the poor and disenfranchised.

Socio-economic factors - not social networking - are fuelling a large part of the obesity epidemic.

You can't just blame the company people keep. You have to look at the society they live in.

That truly is a collective, not an individual, problem.