According to researchers in the U.S. a common virus (adenovirus-36), that causes throat and eye infections may play a part in obesity.
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.