Not the long kind of dying that stretches on for months or years. He would die today. Maybe tomorrow. And if not tomorrow, the next day. Was there someone I should call? Someone he wanted to see?

Not a one, he told me. No immediate family. No close friends. He had a niece down South, maybe, but they hadn’t spoken in years.

For me, the sadness of his death was surpassed only by the sadness of his solitude. I wondered whether his isolation was a driving force of his premature death, not just an unhappy circumstance.

Every day I see variations at both the beginning and end of life: a young man abandoned by friends as he struggles with opioid addiction; an older woman getting by on tea and toast, living in filth, no longer able to clean her cluttered apartment. In these moments, it seems the only thing worse than suffering a serious illness is suffering it alone.

Social isolation is a growing epidemic — one that’s increasingly recognized as having dire physical, mental and emotional consequences. Since the 1980s, the percentage of American adults who say they’re lonely has doubled from 20 percent to 40 percent.

About one-third of Americans older than 65 now live alone, and half of those over 85 do. People in poorer health — especially those with mood disorders like anxiety and depression — are more likely to feel lonely. Those without a college education are the least likely to have someone they can talk to about important personal matters.

A wave of new research suggests social separation is bad for us. Individuals with less social connection have disrupted sleep patterns, altered immune systems, more inflammation and higher levels of stress hormones. One recent study found that isolation increases the risk of heart disease by 29 percent and stroke by 32 percent.

Another analysis that pooled data from 70 studies and 3.4 million people found that socially isolated individuals had a 30 percent higher risk of dying in the next seven years, and that this effect was largest in middle age.

Loneliness can accelerate cognitive decline in older adults, and isolated individuals are twice as likely to die prematurely as those with more robust social interactions. These effects start early: Socially isolated children have significantly poorer health 20 years later, even after controlling for other factors. All told, loneliness is as important a risk factor for early death as obesity and smoking.

The evidence on social isolation is clear. What to do about it is less so.

Loneliness is an especially tricky problem because accepting and declaring our loneliness carries profound stigma. Admitting we’re lonely can feel as if we’re admitting we’ve failed in life’s most fundamental domains: belonging, love, attachment. It attacks our basic instincts to save face, and makes it hard to ask for help.

I see this most acutely during the holidays when I care for hospitalized patients, some connected to I.V. poles in barren rooms devoid of family or friends — their aloneness amplified by cheerful Christmas movies playing on wall-mounted televisions. And hospitalized or not, many people report feeling lonelier, more depressed and less satisfied with life during the holiday season.

New research suggests that loneliness is not necessarily the result of poor social skills or lack of social support, but can be caused in part by unusual sensitivity to social cues. Lonely people are more likely to perceive ambiguous social cues negatively, and enter a self-preservation mind-set — worsening the problem. In this way, loneliness can be contagious: When one person becomes lonely, he withdraws from his social circle and causes others to do the same.

Dr. John Cacioppo, a psychology professor at the University of Chicago, has tested various approaches to treat loneliness. His work has found that the most effective interventions focus on addressing “maladaptive social cognition” — that is, helping people re-examine how they interact with others and perceive social cues. He is collaborating with the United States military to explore how social cognition training can help soldiers feel less isolated while deployed and after returning home.

The loneliness of older adults has different roots — often resulting from family members moving away and close friends passing away. As one senior put it, “Your world dies before you do.”

Ideally, experts say, neighborhoods and communities would keep an eye out for such older people and take steps to reduce social isolation. Ensuring they have easy access to transportation, through discounted bus passes or special transport services, can help maintain social connections.

Religious older people should be encouraged to continue regular attendance at services and may benefit from a sense of spirituality and community, as well as the watchful eye of fellow churchgoers. Those capable of caring for an animal might enjoy the companionship of a pet. And loved ones living far away from a parent or grandparent could ask a neighbor to check in periodically.

But more structured programs are arising, too. For example, Dr. Paul Tang of the Palo Alto Medical Foundation started a program called linkAges, a cross-generational service exchange inspired by the idea that everyone has something to offer.

The program works by allowing members to post online something they want help with: guitar lessons, a Scrabble partner, a ride to the doctor’s office. Others can then volunteer their time and skills to fill these needs and “bank” hours for when they need something themselves.

“In America, you almost need an excuse for knocking on a neighbor’s door,” Dr. Tang told me. “We want to break down those barriers.”

For example, a college student might see a post from an older man who needs help gardening. She helps him plant a row of flowers and “banks” two hours in the process. A few months later, when she wants to cook a Malaysian meal for her boyfriend, a retired chef comes by to give her cooking lessons.

“You don’t need a playmate every day,” Dr. Tang said. “But knowing you’re valued and a contributing member of society is incredibly reaffirming.”

The program now has hundreds of members in California and plans to expand to other areas of the country.

“We in the medical community have to ask ourselves: Are we controlling blood pressure or improving health and well-being?” Dr. Tang said. “I think you have to do the latter to do the former.”