Loneliness is a fundamental part of the human experience. It is as raw and universal an emotion as sadness, anger, and happiness, and it is felt as strongly as hunger and thirst.

But loneliness “is not synonymous with being alone, nor does being with others guarantee protection from feelings of loneliness,” says John Cacioppo, PhD, Tiffany and Margaret Blake professor at the University of Chicago and director of the Center for Cognitive and Social Neuroscience. Rather, loneliness is dependent on the way we perceive our social situations. We can be alone and feel relaxed and peaceful, or we can be in a crowd of people and feel utterly disconnected.

Therefore, loneliness stems from a want of—and lack of—intimacy with other humans.

Some people only need one person in their lives to meet this requirement for emotional intimacy, while others need more. Dr. Cacioppo asserts that introverts usually need just one friend to feel connected, while extroverts need at least three.

So what is the consequence of having our desire for human connection go unfulfilled?

Many scientists, psychologists, and researchers have found loneliness to be a greatly contributing factor to mental and physical illness and even premature death. Loneliness has been linked to obesity, diabetes, high blood pressure, heart disease, dementia and more.

Loneliness root of mental illness

Frieda Fromm-Reichmann, a famous therapist who cured the supposedly incurable Joanne Greenberg from her severe schizophrenia, used a non-medication approach to help her patients. She believed that no patient, including Greenberg, was too far-gone to be healed through trust and intimacy, and thought that loneliness was the root cause of all mental illnesses. After Fromm-Reichmann’s therapy, Greenberg went on to college and to write a bestseller called I Never Promised You a Rose Garden, a fictionalized telling of her experience in the loving care of Fromm-Reichmann.

Loneliness could also lead to depression, cognitive decline and the onset of dementia in high-risk individuals, as suggested by many a study.

Loneliness sparks physical disrepair

Similar to mental illness, loneliness could affect physical health, and more than we might initially think. A meta-analysis of over 100,000 subjects published in 2012 discovered that the effects of social connection on health were about three times larger than the effects of obesity on health. Other research found that those who are not lonely tend to recover faster from illnesses, while those who are lonely experience more complications and a longer recovery.

Loneliness occurs more in older individuals, and increases more as the decades go by. A 2010 survey published by AARP found that a little more than one of three adults 45 and older were chronically lonely, when this percentage was one out of five a decade earlier. Those who were married and had higher incomes were less likely to be lonely than single people with lower salaries. The survey discovered that loneliness was a significant predictor of poor health.

Loneliness may cause death

Loneliness could even lead to premature death. A 2012 study observed people 60 and older for six years and found they were 45 percent more likely to die from health problems if they felt lonely, isolated or left out. But this loneliness wasn’t necessarily connected to whether the person lived alone or not.

Poor health from loneliness could be due to the feelings themselves that hurt the body, as Dr. Cacioppo argues, or due to the failure of social frameworks—the lonely may get sicker because they don’t have people to take care of them.

Survival mode turned on

Whatever the reason, poor mental and physical health from loneliness goes all the way back to our evolutionary and primal roots. When we experience loneliness, we experience more stress as our bodies are placed in survival mode—in ancient times, staying close to others meant protection, shelter, and food.

“Today when we feel left out, our bodies may sense a threat to survival, and some of the same pain signals that would engage if we were in real physical danger are flipped on,” Sanjay Gupta, MD, neurosurgeon and assistant professor of neurosurgery at Emory University School of Medicine, wrote in O Magazine. “In the chronically lonely, levels of the stress hormone cortisol shoot up higher in the morning than in more socially connected people and never fully subside at night.”

This surge in stress hormones wears on our bodies after time, perhaps so slowly that we don’t even notice until we are facing mental or physical disease.

We need more meaningful interaction

So how can we combat loneliness and its resulting health problems? More social interaction with people you can count on.

“Our own work suggests that what’s important is having friends on whom you can count. Popular people and billionaires have more than enough friends, but they can be very lonely because they can’t trust anyone,” Dr. Cacioppo said in an interview.

Social media and other online communication can be a good addition to face-to-face contact by helping create or strengthen friendships that lead to face-to-face interaction, he says, but it should not substitute for spending time with humans in the same physical space. We should seek out meaningful friendships in the old-fashioned style of physical interaction.

And, take note of the times when you feel lonely as a way of nature giving you a warning sign that you need more loving attention. “Physical pain motivates us to take care of our physical body. Loneliness motivates us to take care of our social body, and in doing so, it fosters caring about others and being willing to work to stay together. We’re a fundamentally social species, and a social animal that is isolated is almost certain to live a shorter, more miserable life.”