Loneliness, like the flu, is contagious, U.S. research shows.

It can spread among groups of people and women are more likely than men to become "infected," according to researchers at the University of Chicago, the University of California-San Diego and Harvard.

Using data from a large-scale study, they found lonely people tend to transmit their sad feelings to those around them, which eventually led to them being isolated from society.

"We detected an extraordinary pattern of contagion that leads people to be moved to the edge of the social network when they become lonely," said University of Chicago psychologist John Cacioppo, a leading U.S. expert on loneliness.

The findings were published in the December issue of the Journal of Personality and Social Psychology.

Before losing their friends, lonely people transmit feelings of loneliness to their remaining friends, who also become lonely.

"On the periphery people have fewer friends, yet their loneliness leads them to losing the few ties they have left," Cacioppo said.

"These reinforcing effects mean that our social fabric can fray at the edges, like a yarn that comes loose at the end of a crocheted sweater," Cacioppo added.

Because loneliness is associated with mental and physical diseases that can shorten life, Cacioppo said it is important for people to recognize loneliness and help those affected before they move away to the edges.

For the study, the team examined records of the Framingham Heart Study, which originally studied the risks of cardiovascular disease for more than 5,000 people since 1948.

The study has since been expanded, and its second generation, which includes another 5,124 people, was the focus of the loneliness research.

The study showed that as people become lonely, they become less trustful of others, and a cycle develops that makes it harder for them to form friendships.

Societies seem to develop a natural tendency to shed these lonely people, something that is mirrored in tests of monkeys, Cacioppo said, adding that this makes it all the more important to recognize loneliness and deal with it before it spreads.