Researchers believe that an airborne “adenovirus” germ could be causing the fat plague that is blighting Britain and other countries.
As many as one in three obese people may have become overweight after falling victim to the highly infectious cold-like virus, known as AD-36.
It is known to cause coughs, sore throats, diarrhoea and conjunctivitis but has now also been found to make fat cells multiply, leading to weight gain.
The shocking discovery will add to evidence that Britain’s obesity epidemic is not simply down to an unhealthy diet or lack of exercise.
Research suggests a third of UK adults will be grossly overweight within three years, with Britain even predicted to overtake the US as the most obese nation in the world.
The problem already causes 9,000 premature deaths in the UK a year and costs the NHS £1billion.
Many experts already believe that genes can make some people more susceptible to weight gain and now it seems that infections could also hold the key.
Studies have shown that chickens and mice infected with AD-36 put on weight quicker than uninfected animals, even when they were not given extra food. It has also been found to cause huge weight gain in monkeys.
Now studies on humans show that 33 per cent of obese adults had contracted AD-36 at some point in their lives, compared with only 11 per cent of lean men and women.
Professor Nikhil Dhurandhar, of Pennington Biomedical Research Centre in Louisiana, US, who led the research, said AD-36 continued to add weight gain long after those infected had seemingly recovered.
His studies indicated that the virus lingers for up to three months, during which time it multiplies fat and is contagious to others.
Dr Dhurandhar, who will make the extraordinary claims on BBC2’s Horizon tonight, said: “We now know that this virus goes to the lungs and spreads to various organs such as the liver, kidney, brain and fat tissue.
“When it goes to fat tissue it replicates, making more copies of itself and in the process increases the number of new fat cells, which may explain why people get fat when they are infected with this virus.” The findings were welcomed by some medical experts, although others sounded a note of caution.
Dr Shahrad Taheri, clinical director for obesity at the Birmingham Heartlands Hospital, said: “Most people believe obesity is caused by environmental factors.
“But there is a lot of information about how things like the furring up of arteries could be linked to infections. It is not beyond reason to think about various different factors, including infections, adding into the mix about what causes obesity.”
Tony Barnett, professor of medicine at the University of Birmingham, said: “These associations may give some clues but they detract from the basic message that we all need to take more exercise and eat a bit less.
“This kind of research needs to go on but we have to be cautious.”
Dr Colin Waine, chairman of the National Obesity Forum, said: “We must acknowledge it is a contribution to the research but it doesn’t alter the management of obesity.”
The documentary also reveals research which claims to explain why those on diets feel permanently hungry, even when overweight.
A US study found that people have a “natural body weight” and respond to losing a few pounds in the same way as if they were starving.
The findings suggest that overweight people who diet will always suffer hunger pangs, even if they become lean and healthy.
* Horizon: Why Are Thin People Not Fat? is on BBC2 at 9pm tonight.