Researchers in the United States say loneliness, like the flu, is contagious.
They say it can spread among groups of people, and women are more likely than men to become infected.
The researchers, from three American universities, found lonely people tend to transmit their sad feelings to those around them, which eventually led to them being isolated from society.
Before losing their friends, lonely people transmit feelings of loneliness to their remaining friends, who also become lonely.
The findings were published in the December issue of the Journal of Personality and Social Psychology.