A new study has shown that while friends can sometimes be priceless when it comes to criticizing your mistakes, they can also become sort of an enemy when it comes to getting fat.

The breakthrough study is probably the first that establishes a link between the social component of our every day life and the process of getting fat. Researchers have found that obesity can spread from one member of a group to another just like a virus, and the highest risk is associated with the closest friends. These can triple your risk of becoming fat.

"We were stunned to find that friends who live hundreds of miles away have just as much impact as friends who are next door," said James Fowler, one of the study's authors and an associate professor of political science at the University of California San Diego (UCSD). The study at which he participated, conducted by Dr. Nicholas A. Christakis of Harvard Medical School, is published in the latest issue of New England Journal of Medicine.

The recent findings suggest obesity has a spreading pattern similar to AIDS or influenza inside social networks, potentially warning physicians over the “infectious” character of the disease.

"It's about the spread of norms from person to person," said Christakis, a professor of medical sociology. The research was based on more than 12,000 people taking part in the three-decade-long Framingham Heart Study.

At each update in that study, doctors monitored participants' height and weight and also recorded information about their neighbors, friends and spouses.

Using new software, Harvard and UCSD researchers created diagrams that plotted obesity and relationships, mapping the past 30 years.

The study also found that people who are thinner tend to influence their peers and relatives in the same way fat people influence their friends: if someone sees a thin person, he/she might start losing weight.

"It is very plain to those of us who work in community settings that health behaviors occur in the context of a social network," said Dr. Katherine Kaufer Christoffel, a childhood obesity expert at the Northwestern University Feinberg School of Medicine who was not connected with the research.

"This is a seminal study," said Richard Suzman, director of the National Institute on Aging's behavioral and social research program, which funded the research. "It takes what was seen as a noninfectious disease and shows it clearly has got communicable factors."