Researchers believe that the highly-infectious adenovirus, which can be spread by coughs or through dirty hands, attacks tissue and causes fat cells to multiply - causing weight gain.
Scientists from Pennington Biomedical Research Centre in Louisiana found that chickens and mice infected with the virus gained weight more quickly than uninfected animals - even when eating the same amount. Studies on humans have shown that almost a third of obese adults carry the virus compared with 11 per cent of men and women who are not obese.
Professor Nikhil Dhurandhar, who led the research, told BBC2's Horizon programme, to be shown at 9pm tonight: "This virus goes to the lungs and spreads through the body. "It goes to various organs and tissues such as the liver, kidney, brain and fat tissue.
"When this virus 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 the fat tissue expands and why people get fat when they are infected with this virus." Professor Dhurandhar said the bug continues to add weight gain long after those infected recover from their cough or cold and said victims could remain infectious for up to three months.
He added: "But people could be fat for reasons other than viral infections so it's really pointless to try to avoid fat people to avoid infection." Dr Dhurandhar believes that a vaccine aimed at this kind of "infectobesity'' could be ready for tests within five or 10 years. But British obesity experts said any evidence of there being a link between obesity and adenovirus was "sparse".
Dr Ian Campbell, a GP and medical director of the charity Weight Concern, said: "A virus will never be the reason for why we have an obesity epidemic. "There are far too many other factors, starting with our calorie intake exceeding our expenditure, and that's because we live such sedentary lives. "Our dietary habits have changed beyond belief and I don't believe that's the effect of a viral infection - it is the fault of the commercial expansion of companies making unhealthy foods."
Professor Colin Waine, past president of the National Obesity Forum, added: "What we don't want to lose sight of is that if people can lose 5 to 10 per cent of their weight, the benefits on health are disproportionately good." Tam Fry, of the Child Growth Foundation, said: "You are much more likely to pick up the flu than obesity. In general, obesity is down to eating more than you need and not exercising as much as you should."
Tonight's Horizon documentary, called Why Are Thin People Not Fat? also features research into why people on diets always feel hungry and often regain the weight they have lost. Dr Rudy Leibel, from Columbia University in New York, said individuals have a "natural body weight" to which they are programmed to return after dieting.
"Individuals have a biology which determines how tall or short they will be and how skinny or fat they will be, and wishing it one way or the other really cannot change it that much."