The scientists from Pennington Biomedical Centre at Louisiana State University, conducted laboratory tests on human fatty tissue and found that the virus triggered changes in the tissue that left people with more, and larger, fat cells than people who were not infected.

The scientists say they recognise that the virus may be just be one contributing factor in obesity but they suggest an understanding of how fat cells respond to infection could lead to vaccines to prevent weight gain.

They also say the research could possibly lead to fat-promoting treatments for people with rare wasting conditions, such as lipidystrophy.

The study found that the infection was three times more common in obese people and may explain the "healthy obese", who are substantially overweight but have healthy blood pressure and cholesterol levels.

The study continues the line of research which linked the virus to animal obesity but is the first to suggest infection may drive weight gain in humans.

Researchers Dr. Magdelena Pasarica and Dr. Nikhil Dhurandhar collected fatty tissue from 10 patients having liposuction from which they extracted stem cells.

Half of the tissue of each patient was then exposed to adenovirus-36, a cause of respiratory tract infections, diarrhoea and conjunctivitis; they found over a period of one week that many of the stem cells exposed to the virus had been converted into early stage fat cells and begun storing fat.

In samples that were not infected, only a third as many stem cells had become fat cells.

Dr. Pasarica says the study provides strong evidence that some obesity cases may involve viral infections.

Dr. Dhurandhar says the virus adenovirus-36 or Ad-36, caused animals to pack on the pounds in earlier lab experiments and the animals accumulated far more fat.

Dhurandhar also has shown that obese people were three times more likely to have been infected with Ad-36 than thin people in a large study of humans.

He says some people might find it hard to believe that a virus could be responsible for obesity and says there are multiple causes of obesity, ranging from simple overeating to genetics, to metabolism and also perhaps viruses and infections.

According to the World Health Organization globally around 400 million people are obese, including 20 million children under age 5.

Dr. Pasarica presented her findings at a meeting of the American Chemical Society in Boston.