Obesity is Contagious

Not via a microbe, but via social norms. The study showed that if you have a friend who becomes obese, you're much more likely to become obese.

The researchers hypothesize that the same should hold true of losing weight. That is, if you have a friend who loses weight, you're more likely to lose weight. Thus, it helps to lose weight with friends who are doing the same. Trying to be the only one in your social circle who is losing weight will be very difficult.