For all of us looking for another excuse for putting on a few pounds, we've got a whopper.

"Obesity spreads through friends and through spouses and through siblings," says James Fowler of the University of California, San Diego.

The study says obesity has become a "socially contagious" disease — spreading person to person almost like a virus.

Among married couples, when one spouse becomes obese, the risk to the other increases by 37 percent. For siblings, the risk rises 40 percent.

So, while everyone might have gotten a good laugh watching "The Nutty Professor" say "that certain people are genetically predisposed to gain weight," the joke may have been on us.

Nobody knows this better than Keisha and Shavon Brown, two sisters who used to be a bad influence on each other.

"When I said I didn't want to work out," says Shavon, "she said she didn't either, and we would stay home."

Lazy lifestyles and super-sized portions are partly to blame. But researchers say it's also because we look at our friends for an idea of normal body size.

"If the social norm is to be at a higher weight, then other people accept their body weight at a higher level," says Dr. Lou Aronne, a specialist on weight loss and obesity.

The study also found the strength of the relationship also matters.

Among casual friends, when one becomes obese, the risk for the other increases 57 percent. For close friends, the risk nearly triples.

The good news is that this research suggests if friends can help you get fat, they should also be able to help you get thin — for example, by sharing time at the gym together instead of sharing a cheeseburger.

The Brown sisters did just that. With the guidance of their life coach, they are eating healthier and working out more.

So far, they have lost 40 pounds combined. So either follow their example, or keep your heavier friends close — but your thinner enemies closer.