By Diane Roberts
BBC News health producer
Waist measurement is one of the best ways of establishing obesity
Could it really be possible to catch obesity?
One doctor in the US believes that a common cold virus can interfere with our normal body processes and make us fat.
For the past ten years, Dr Nikhil Dhurandhar from Louisiana has been carrying out animal and human studies on the virus, Adenovirus-36.
He believes it could be one of the mechanisms causing some people to put on weight more quickly.
His team at the Pennington Biomedical Research Centre have now documented more than 1,000 patients whose obesity appears to be linked to infection with the virus.
Dr Dhurandhar's team took blood samples from people an obesity clinic.
They tested it for antibodies to Adenovirus-36 and found that 20% of the patients had encountered the virus at some point and were significantly heavier than their antibody negative counterparts.
In another study, they showed that obese people were nearly three times more likely to have the virus than a non obese person and even amongst the non obese group, those with the virus were heavier than average.
One in three adults in the UK will be obese by 2012
By 11 years of age, 33% of children in the UK are overweight or obese
9,000 adults in the UK die early every year because of obesity
Dr Dhurandhar said: "When this virus goes to the fat tissue it replicates making more copies of itself and in the process increases the number of fat cells, which may explain why people get fat when infected with this virus."
So if we want to remain slim, should we be shunning fat people?
Dr Dhurandhar said we probably encounter it first when we are small children and only remain infective for two to three months and there are many other reasons why we get fat.
And Dr Carel Le Roux, an obesity expert at Imperial College, who has been carrying out experiments to see if he can make thin people fat, said: "It's very important to know that it's not the reason why we're seeing a major epidemic of obesity.
"It may be a small contributing factor and we need to explore all the avenues because so many people need help and we're just not clever enough to help them at the moment."
The World Health Organisation says that globally there are more than a billion overweight adults and a third of them are obese.
Obesity rates that have risen three-fold or more since 1980 in some areas of North America, the United Kingdom, Eastern Europe, the Middle East, the Pacific Islands, Australasia and China.
But Dr Tam Fry, chair of the Child Growth Foundation, said: "I'm sceptical because this theory has been around for 10 years and no-one has come up with a comparable study to back this up.
"Concern over the obesity epidemic seems to be throwing up a whole load of off-the-wall ideas but the message remains the same, that sensible eating and exercise are the major components to get your weight under control."
Horizon: Why are thin people not fat? was broadcast on BBC Two on Mon 26th Jan at 9pm. It is available in the UK on BBC iplayer for 7days.