Watch out if your best friend gets pudgy.

A new study suggests obesity can spread like an infectious disease and that your odds of becoming obese are much greater if your friends and family put on weight.

By studying a large social network of 12,067 people who have been closely tracked for the past three decades, researchers found that when one person became fat, those close to them gained as well.

The strongest influence was seen among friends no matter where they lived. A person's chances of becoming obese went up 57 percent if a friend became obese. In the closest friendships, the risk almost tripled. Among siblings, the risk of obesity increased by 40 percent and 37 percent among spouses.

The researchers think it's more than just people with similar eating and exercise habits hanging out together. Instead, it may be that having relatives and friends who become obese changes one's idea of what is an acceptable weight.

Despite their findings, the researchers said people should not sever their relationships.

Researchers analyzed medical records of people in the Framingham Heart Study, which has been following the health of residents of that Boston suburb for more than a half century. They tracked records for relatives and friends using contact information that participants provided each time they were examined over a 32-year period.

In all, 12,067 people - all Framingham participants - were involved in the study.

After taking into account natural weight gain and other factors, researchers found that social ties seemed to play a surprisingly strong role, even more than genes are known to do. Geography and smoking cessation had no effect on obesity risk.