Updated at 3:35 p.m. ET, Tuesday, March 6.

Loneliness can send a person down a path toward bad health, and even more intense loneliness, studies have shown. But while some have assumed the culprit was a dearth of others to remind a person to take care of himself or herself, new research suggests there's a direct biological link between being lonely and ill health.

Loneliness can set into a motion a barrage of negative impacts inside the human body — but with additional social contact, some of the ill effects can be stopped.

John Cacioppo, a University of Chicago social psychologist who studies the biological effects of loneliness, presented some of his latest research at the Society for Personality and Social Psychology meeting in San Diego in January.

He has found, for instance, loneliness is tied to hardening of the arteries (which leads to high blood pressure), inflammation in the body, and even problems with learning and memory. Even fruit flies that are isolated have worse health and die sooner than those that interact with others, showing that social engagement may be hard-wired, Cacioppo said.

In one study, Cacioppo and Steve Cole of UCLA examined how the immune system changed over time in people who were socially isolated. They observed a change in the kinds of genes that lonely people's immune systems were expressing. Genes overexpressed in the loneliest individuals included many involved in immune system activation and inflammation. In addition, several key gene sets were underexpressed, including those involved in antiviral responses and antibody production. The result is that a lonely person's body has let its defenses down to viral and other invaders. [7 Personality Traits That Are Bad For You]

"What we see is a consistent pattern where it looks like human immune cells are programmed with a defensive strategy that gets activated in lonely people," Cole told LiveScience.

Here's why: The immune system has to make a decision between fighting viral threats and protecting against bacterial invasions because it has a fixed fighting capability. In lonely people who see the world as a threatening place, their immune systems choose to focus on bacteria rather than viral threats. Without the antiviral protection and the body's antibodies produced against various ills, the result means a person has less ability to fight cancers and other illnesses. Those who are socially isolated suffer from higher all-cause mortality, and higher rates of cancer, infection and heart disease.

In addition, loneliness raises levels of the circulating stress hormone cortisol and blood pressure, with one study showing that social isolation can push blood pressure up into the danger zone for heart attacks and strokes. It undermines regulation of the circulatory system so that the heart muscle works harder and the blood vessels are subject to damage by blood flow turbulence. Loneliness can destroy the quality of sleep, so that a person's sleep is less restorative, both physically and psychologically. Socially isolated people wake up more at night and spend less time in bed actually sleeping, according to Cole and Cacioppo's research.

The cycle created by loneliness can be a downward spiral. Studies by Cacioppo and others before him have found that lonely people tend to rate their own social interactions more negatively and form worse impressions of people they meet.

"Much like the threat of physical pain, loneliness protects your social body. It lets you know when social connections start to fray, and causes the brain to go on alert for social threats," Cacioppo told LiveScience. "Being lonely can produce hyper-reactivity to negative behaviors in other people, so lonely people see those maltreatments as heavier. That makes it possible to fall more deeply into loneliness."

The reasons trace back to humanity's evolutionary history, when people needed each other to stay alive. Loneliness doesn't just make people feel unhappy, it actually makes them feel unsafe — mentally and physically. This powerful evolutionary force bound prehistoric people to those they relied on for food, shelter and protection, to help them raise their young and carry on their genetic legacy. Cacioppo surmises that the distress people feel when they drift toward the edges of a group serves as a warning — like physical pain — that they need to reengage or face danger.

Everyone feels left out for some period of time, be it moving to a new city or starting college. Typically the feelings subside by themselves within six months. But when it comes to treating chronically isolated people, some interventions work better than others. In a large meta-analysis done last year, Cacioppo and colleagues found that two of the best ways to treat loneliness are to train people for the social skills they need to view the world in a more positive light, and to bring people together to share good times.

Correction: This article has been updated to correct the name and timing of the Society for Personality and Social Psychology meeting (not the Social Psychology and Perception meeting), which occurred in January, not February as we had written.

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