There's more evidence linking obesity to a virus.
Researchers from Louisiana State University have found that when human stem cells were exposed to a common virus they turned into fat cells and then got bigger.
While this may explain why some people put on weight, it doesn't explain most of America's growing obesity problem.
Researchers took fat tissue from people who had liposuction, removed adult stem cells and exposed the cells to a virus in the lab. There, more than half the cells turned into fat cells and also accumulated fats.
In the end, if a viral cause of obesity can be shown, a vaccine could be developed to prevent the virus from making people heavy. But it wouldn't help people who already are obese.