When the weight problem in the United States is referred to as an epidemic, it isn't meant to imply that obesity is contagious. A recent study in the New England Journal of Medicine, however, reports that it is indeed possible to "catch" obesity from others.
In particular, if someone's close friend becomes obese, there's a 171% chance that he/she will become obese as well.
Surprisingly, siblings and spouses were found to have much less of an influence on each other.
Before you dump your chubby best bud, keep in mind that you can have a positive influence on others: weight loss was also found to be contagious. (Let's face it -- there's no way in hell they're going to let YOU get all the attention.)