Loneliness is thought to be rising around the world and how often you see friends and family could have a significant effect on health.
Doctors have known for some time that loneliness is bad for the mind. It leads to mental health problems like depression, stress, anxiety, and a lack of confidence.
But there's growing evidence that social isolation is connected with an increased risk of physical ill health as well.
There are suggestions it can make some diseases both more likely to occur and more likely to be fatal.
In 2006, a study of 2,800 women who had breast cancer showed those who saw few friends or family were as much as five times more likely to die of their disease than women with many social contacts.
Researchers are trying to figure out what loneliness does to the body which can lead to illness and death.
Psychologists at University of Chicago and Ohio State University have shown that people who are socially isolated develop changes in their immune system, which leads to a condition called chronic inflammation.
Short term inflammation is necessary for us to heal after a cut or an infection, but if the inflammation persists in the long-term it can contribute towards cardiovascular disease and cancer.
At the University of Chicago, scientists found that lonely people find everyday activities more stressful than those who are not socially isolated.
They measured levels of cortisol, a hormone that's produced when we are stressed, in a wide range of healthy people in the morning and evening.
Lonely people released more cortisol. The scientists suggest that too much of the hormone causes inflammation and disease.
The latest work from Ohio State University looked at levels of inflammation in response to stress in lonely people. Dr Lisa Jaremka compared women who have survived breast cancer with healthy volunteers.
She gave the participants a well-known stress test, called the Trier Social Stress Test, in which they had to give an impromptu speech explaining why they were the best candidate for a job, in front of a stony-faced panel.
They then had to perform a mental arithmetic task before the same panel.
Loneliness tests and blood samples showed that in both groups, the lonelier people had higher levels of inflammation.
Dr Jaremka said: "If you're lonely you can have raised inflammation regardless of having a chronic medical condition.
"It was a struggle for a long time for physicians to recognise the importance of loneliness in health. We now know how important it is to understand patients' social worlds."
The number of people who are likely to be lonely is rising all over the world. Many of these are elderly, left by themselves after their partners have died or their families have moved away.
Half of over 75 year olds in the UK live alone, and one in 10 suffer intense loneliness.
Dr Jaremka said: "Being lonely means not feeling connected or cared for, it's not about being physically alone.
"We need to find ways to help lonely people. Unfortunately we can't tell anyone to go out and find someone to love you. We need to create support networks."