A recent study found chubby friends may increase your chance of being overweight, but could you also catch obesity the same way you get other bugs?

Researchers say some contagious pathogens make fat cells grow larger and multiply faster -- and some packing on the pounds could be oblivious victims of "infectobesity."

U.S. researcher Dr. Nikhil Dhurandhar, who coined the term, has focused his research on a specific adenovirus, known as AD -36.

Studies on the virus have shown humans who test positive for the pathogen are more likely to be obese than those who test negative.

One experiment looked at more than 500 people and found:

• 30 per cent of individuals who were obese had antibodies for the virus;
• 11 per cent of obese subjects did not show the virus;
• The non-obese subjects all had a Body Mass Index of less than 30 per cent
• Those who tested positive were generally heavier than the rest of the group.

A study on twins also found if one tested positive and the other negative, the one with AD -36 weighed more, on average, than the other.

Dhurandhar, who became curious about the possible link when a colleague studying a bird virus noticed infected chickens had a lot of fat, said AD -36 isn't the only microbe that could be contributing to expanding waistlines.

"There are 10 different pathogens that have been shown to cause obesity in different animal models by different research groups," Dhurandhar said.

How could a virus make people fat?

Dr. Richard Atkinson, another researcher in the field, said AD -36 increases body fat by causing undifferentiated cells, or stem cells, to turn into fat cells faster than they normally would.

The virus then tells the cells to produce a lot of lipids, or fats. The result: bigger fat cells -- and more of them.

"The virus turns on factors, like enzymes, that cause new fat cell formation," Atkinson said.

"An infected fat cell would then secrete these things, and that would kick off the cells in the neighbourhood that have the potential to turn into fat cells."

Atkinson said although scientists can't ethically give humans AD -36, the virus has caused weight gain in almost all the animals researchers have infected -- including all of the monkeys, who are considered the closest model for humans.

He says the data shows getting the virus pretty much spells weight gain, even if your lifestyle doesn't change.

"There are a whole lot of people who will say people just do it to themselves -- they're just eating too much and not exercising," Atkinson said.

"What we can say is 'wait a second -- if I give you this virus, you will get fat, even if you're doing exactly what you've been doing.'"

If you're worried about the bulge...

For US$450, Obetech, a company run by Atkinson, will test your blood and tell you if you've been infected by AD -36, but the virus can't be cured -- so why would people bother bucking up?

Atkinson said the test can help lean people avoid a fat fate.

"For the person who is not yet fat or who has just started to gain weight ... if you turn up with antibodies, your chances of getting fat are very high if you don't treat it," Atkinson said.

He said knowing you've got the virus can help keep the pounds off because those who test positive know they'll probably have to make some lifestyle changes to stay trim.

"Compare to cholesterol test. If your doctor says 'wow, your cholesterol is up', you can say 'oh well, I'll just have a heart attack and die'," Atkinson said.

"Or you can diet, you can exercise, you can take drugs to lower your cholesterol, and maybe put that heat attack off forever."

If you're already overweight...

Atkinson also said the test can help obese people understand why they're fat.

"I've seen many people who say 'I was always skinny and then I suddenly gained 75 pounds, what's the matter with me?'" He said.

Atkinson explained if AD -36 positive people find the virus could have made them obese, they can better understand why they're heavy.

"Studies have shown that all people; not just skinny people, but fat people too, think that obese people are lazy, they don't have any self-discipline and they can't be trusted," Atkinson said.

"If a person takes that to heart and says 'oh yeah, I'm a terrible person', then they come along and they find 'I'm fat because I'm infected with a virus' ... 'I just happened to be at the wrong place at the wrong time,' they realize they're not a bad person."

Is obesity a disease?

Dr. Rena Mendelson, nutrition section head for the Canadian Obesity Network, said most people in the medical field know obesity a disease -- but the debate carries on in part because of treatment costs.

"Obesity is caused by metabolic factors as opposed to lifestyle factors," Mendelson said.

"We're not defining it because we can't afford to cover treatment for all the people who have the disease."

She also said obesity is caused be a multitude of reasons over a population, but a single factor could trigger it in an individual.

Dhurandhar believes pathogens are one of those factors -- he said the idea has driven his passion for research since he first encountered the fat chickens in India two decades ago.

"I've wanted to point out that infections, as a class, maybe also be causal factors in some cases of obesity -- and if 10 have been discovered, imagine how many may be out there," Dhurandhar said.

"Unless we have this concept in the back of our minds we will never go looking for more, and if we don't look, we won't find them."