Loneliness or social isolation, in the long term, is as damaging as a 15-a-day cigarette habit or being an alcoholic.
Many other studies have found that those with a poor social network are at increased risk of dementia and high blood pressure.
The studies reason that the genes we need to fight off serious viral infections seem to be less active in the lonely than in the rest of the population, and that loneliness may cause cancer or heart disease.
"Feeling alone and unloved can also make it harder to sleep and even speed the progression of dementia. When time takes its toll on the body, loneliness steepens that slope of descent," the Daily Mail quoted Chicago-based psychologist John Cacioppo, who has studied the phenomenon, as saying.
His work found that loneliness raises levels of the stress hormone cortisol and can push blood pressure up into the danger zone for heart attacks and strokes.