The obesity explosion that has swept the Western world over the past 30 years may have been caused by a virus, scientists have said.

Researchers have discovered new evidence for an illness they have called "infectobesity" – obesity that is transmitted from person to person, much like an infection. The agent thought to be responsible is a strain of adenovirus, versions of which cause the common cold. It has already been labelled the "fat bug".

There are more than 50 strains of adenovirus known to infect humans but only one, adenovirus 36, has been linked with human obesity.

Now scientists at the University of California, San Diego, have found that children who showed evidence of infection with adenovirus 36 were more likely to be fat. In tests on 124 children aged eight to 18, the virus was present in more than 20 per cent of those who were obese, compared with less than 6 per cent of the rest. Among those infected with adenovirus 36, four out of five were obese.

Children carrying the virus weighed on average almost 50lb more than those who were not. Among the obese children, who accounted for half the total, those with the virus weighed on average 35lb more than the rest.

Jeffrey Schwimmer, an associate professor of clinical paediatrics, who led the study published in the US journal Pediatrics, said: "This amount of extra weight is a major concern at any age, but is especially so for a child. Obesity can be a marker for future health problems like heart disease, liver disease and diabetes. An extra 35lb to 50lb is more than enough to greatly increase those risks.

"This work helps point out that body weight is more complicated than it's made out to be. And it is time that we move away from assigning blame in favour of developing a level of understanding that will better support efforts at both prevention and treatment.

"These data add credence to the concept that an infection can be a cause or contributor to obesity."

The idea of a viral cause for obesity was first raised a decade ago by Nikhil Dhurandhar, now a professor at the Pennington Biomedical Research Centre in Louisiana. He noticed that chickens which died during a flu epidemic in India in the 1980s – caused by an adenovirus – were plump, rather than thin and emaciated as expected.

Further studies revealed that one in five obese people showed signs of adenovirus infections and were on average more than 28lb heavier than people who had never been infected.

Sceptics pointed out that viruses had never been linked with a long-term disorder such as obesity. Dr Dhurandhar said the evidence was as clear as a map of the USA – where the obesity epidemic "has spread like a forest fire from the east coast to the west over the last 20 years".

Laboratory studies showed the virus infects immature fat cells, prompting them to proliferate and grow more quickly. Professor Schwimmer said: "This might be the mechanism for obesity but more work needs to be done."

Tam Fry, a spokesman for the UK National Obesity Forum said: "This is fascinating research which is starting to come out from the observations on chickens 10 years ago.