The adenovirus-36 (Ad 36) has already been implicated as the cause of weight gain in animals, but with this study researchers showed for the first time that it can also cause humans to pile on the pounds.
The findings could accelerate the development of a vaccine or an antiviral medication to help fight the battle of the bulge alongside diet and exercise.
"We're not saying that a virus is the only cause of obesity, but this study provides stronger evidence that some obesity cases may involve viral infections," said Magdalena Pasarica, an obesity researcher at the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge.
A previous study found that almost a third of obese people are infected with the virus compared to around one in 10 of their leaner counterparts.
In laboratory experiments, the Louisana State University researchers found that the bug appeared to promote the formation of fat cells from stem cells.
The team took adult stem cells from fatty tissue left over from patients who had undergone liposuction, a procedure to remove fat, and exposed some of it to Ad-36, leaving the rest untreated.
After a week of growth in tissue culture, most of the virus-infected adult stem cells developed into fat cells, whereas the untreated cells did not.
It's not clear what drives the transformation, how long the virus lingers in the human system or whether its fat-enhancing effect continues after the body has cleared the virus, the researchers said.
A study in animals found that they remained obese up to six months after the infection had cleared.
The Louisiana State University team is working on further studies to try and establish why some people with the virus develop obesity while others don't.
"Not all infected people will develop obesity," said Pasarica. "We would ultimately like to identify the underlying factors that predispose some obese people to develop this virus and eventually find a way to treat it."
Pasarica presented the results of her study at a meeting of the American Chemical Society in Boston.