Obesity spreads much like infectious diseases, particularly with respect to individuals’ social networks, Harvard researchers say.

According to the classic influenza model, interacting with more sick people increases the likelihood of catching the virus, but the number of healthy people present doesn’t affect recovery chances.

“We were interested in applying this traditional infectious diseases framework to look at social contagiousness of obesity,” said Alison L. Hill, the lead author and a graduate student in Harvard’s Program for Evolutionary Dynamics and the Biophysics Program. “Does obesity follow the same dynamics as the flu?”

The study—published yesterday in the journal PLoS Computational Biology—found having more interactions with obese people indeed increases one’s risks of becoming obese. However, having non-obese people around does not make weight loss any easier.

Analyzing data collected over 40 years from the Framingham Heart Study, the researchers evaluated individuals’ risks of gaining or losing weight, then factored in the impact of their social network, which included parents, children, spouses, close friends, and co-workers.

From this data, they estimated that a non-obese individual has a two percent risk of becoming obese each year, and this figure increases on average 0.4 percent with each obese social contact.

Hill said she and her fellow researchers were surprised to see that the contagiousness of obesity has also risen over the years.

The Centers of Disease Control estimated that one third of Americans are currently obese, with another third overweight but not obese, making obesity a serious health concern for Americans. This study predicts that the obesity level may grow beyond 42 percent if it continues to become more infectious, a claim which contradicts some recent projections that the obesity epidemic has reached a peak, Hill said.

“Traditional approaches to studying obesity don’t take into account social factors,” said David G. Rand, co-author and a research scientist in the Harvard Program for Evolutionary Dynamics.

“Our results have serious implications for policy-makers. [They] raise important sociological questions: what’s causing this phenomenon? How can we prevent obesity from becoming more contagious over time?” he said.

Though the Framingham Heart Study only tracks people over the age of 30, Hill hypothesized that the results would be applicable to young adults and children as well, particularly since they are thought to be more easily influenced by peers.

Nicholas A. Christakis, the senior researcher of the paper and Harvard Medical School professor, did not respond to requests for comment.

—Staff writer Helen X. Yang can be reached at hxyang@fas.harvard.edu.