It torments the young and terrorizes the old. It carved “caverns” in Emily Dickinson’s soul and left William Blake “bereaved of light.”

Loneliness, long a bane of humanity, is increasingly seen today as a serious public health hazard. Scientists who have identified significant links between loneliness and illness are pursuing the precise biological mechanisms that make it such a menace, digging down to the molecular level and finding that social isolation changes the human genome in profound, long-lasting ways.

Not only that, but the potential for damage caused by these genetic changes appears comparable to the injuries to health from smoking and, even worse, from diabetes and obesity. The scientists’ conclusion: Loneliness can be a lethal risk. And the United States — which so prizes individuality — is doing far too little to alleviate it.

“In public health, we talk all the time about obesity and smoking and have all these interventions, but not about people who are lonely and socially isolated,” said Kerstin Gerst Emerson, an assistant professor at the University of Georgia’s Institute of Gerontology. “There are really tangible, terrible outcomes. Lonely people are dying, they’re less healthy, and they are costing our society more.”

Psychologist Steve Cole, who studies how social environments affect gene expression, says researchers have known for years that lonely people are at greater risk for heart attacks, metastatic cancer, Alzheimer’s and other ills. “But we haven’t understood why,” he said.

Then last year, Cole and his colleagues at the UCLA School of Medicine, along with collaborators at the University of California at Davis and the University of Chicago, uncovered complex immune system responses at work in lonely people. They found that social isolation turned up the activity of genes responsible for inflammation and turned down the activity of genes that produce antibodies to fight infection.

The abnormalities were discovered in monocytes, a type of white blood cell, produced in the bone marrow, that is dramatically changed in people who are socially isolated. Monocytes play a special immunological role and are one of the body’s first lines of defense against infection. However, immature monocytes cause inflammation and reduce antibody protection. And they are what proliferates in the blood of lonely people.

Such cellular changes, says University of Chicago social neuroscientist John Cacioppo, are a byproduct of human evolution.

Early on, when survival depended crucially on cooperation and communication, social isolation was a huge risk. So evolution shaped the primitive human brain to desire and need social interaction in the same way it shaped the brain to desire and need food.

The pain of loneliness is like the pain of hunger — it’s a biological signal that something is wrong.

“When you get hungry you may not be aware your blood sugar level is dropping, but if you’re driving and you see the golden arches [of McDonald’s],” you’ll pull in for food, Cacioppo said.

Today, social isolation is often an unavoidable lifestyle. But it puts the body, on the cellular level, on constant alert for a threat. That helps explain why lonely people are more likely to act negatively toward others, which makes it that much harder for them to forge relationships.

“I do see these patients all the time,” said psychiatrist Jacqueline Olds, who has a private practice in Cambridge, Mass., and has co-written two books on the subject. “Many of the people who end up lonely give off signals they want to be alone out of anxiety. . . . Feeling left out has a huge effect on our psyche from our evolutionary worries that everyone else will survive and we won’t.”