To stay healthy, many of us know to eschew cigarettes, moderate alcohol intake, follow a sensible diet, and exercise regularly.But soon, your doctor might also ask: "How many people do you see in a typical week?" That's because how socially connected we are – whether we feel like we're part of a community and cared for by its members – can have a huge impact on our mental and physical health.Video above: Surgeon General warns of loneliness epidemic In a 2015 meta-analysis of 70 studies, people who were socially isolated or lonely were respectively 29 and 26% more likely to die prematurely compared to those who were engaged in their communities. And the effect persisted after accounting for other potential causes – including age, access to health care, and socioeconomic status. In fact, one study equated this increased risk in early mortality to smoking 15 cigarettes a day.Two sides of the same coin, both social isolation (the lack of contacts in your social circle) and loneliness (the subjective feeling of being alone) are associated with the onset of chronic conditions like heart disease, Type 2 diabetes, dementia, and depression. They also heighten our susceptibility to infectious diseases such as colds and COVID-19 and weaken our response to vaccinations.The science of social isolation and loneliness"Humans are a social species," Julianne Holt-Lunstad, a psychologist at Brigham Young University, told Popular Mechanics. "Throughout our history, we've needed to rely on others for survival."And because we've adapted to expect contact with others, losing that connection can lead to the dysregulation of multiple biological systems. This dysregulation shows up in a variety of biomarkers long tied to health or lack of it – from body mass index to waist circumference to blood pressure. Moreover, these associations hold across different age groups and are dose-dependent—the more social contacts an individual has, for example, the lower their blood pressure.For Steven Cole, a genomics researcher at UCLA, the key to many of social isolation’s ills lies in brain-immune system crosstalk, which can push our bodies into a pro-inflammatory state. In the hunter-gatherer era, Cole explains to Popular Mechanics, being alone would have made us more vulnerable to attack and at the mercy of bacteria that can colonize the resulting injuries. Our sympathetic nervous system evolved to meet this threat, pumping norepinephrine, the flight-or-fight hormone, into our tissues.Bone marrow, which produces white blood cells, is one such tissue. In response to the flood of norepinephrine, it ramps up the production of the first-line-of-defense immune cells such as neutrophils and monocytes – at the expense of T-cells and B-cells, which target human-transmissible pathogens. Problems arise, however, when social isolation – and inflammation – persists."This chronic inflammation then acts as an accelerant for diseases like cancer, neurodegenerative diseases, atherosclerotic plaques, and heart attacks," Cole said. And it shows up in our blood. "People who are experiencing isolation have higher levels of inflammatory markers like c-reactive protein and IL-6," Thomas Cudjoe, a geriatrician at Johns Hopkins, told Popular Mechanics.At the same time, changes to gene expression continue to push this inflammatory agenda. For example, Cole and his colleagues found that the white blood cells of lonely people tend to turn on genes involved in making pro-inflammatory cytokines and turn off genes responsible for keeping cell proliferation in check.Trends over timeAccording to most metrics – such as time spent alone and number of social connections –society as a whole, particularly in the developed world, is becoming more isolated. The trend is especially pronounced among adolescents and young adults."For people between the ages of 15 to 24, time spent in person with friends has declined by nearly 70% over the last two decades from 150 minutes per day in 2003 to 40 minutes per day in 2020," Cudjoe says.Although the COVID-19 pandemic highlighted the issue for many, this drift toward a less-connected society has been building for decades. Our modern lifestyle, with its many conveniences, has made it increasingly easy to disengage from others – as anyone with a smartphone and a Doordash or Instacart account can attest."We're also working more hours than ever and often, in a busy schedule, our social connections can get squeezed out," said Holt-Lunstand. The insight that social isolation can kill first struck Cudjoe while treating one of his earliest patients – a medically frail woman, who was desperately lonely, even though she lived in a senior apartment building teeming with other people. During home-health visits "it became difficult for us to even leave because she kept engaging us in conversation," Cudjoe says.Certain groups experience loneliness and isolation, and their attendant consequences, at higher rates compared to the general population. For example, recent immigrants contending with language barriers, the marginalized whose perspectives are ignored, and people facing economic insecurity tend to be at higher risk for isolation, Cudjoe explains.A public health solutionAs the consensus around the dangers of social isolation builds, medical and government institutions are taking notice. In May, the U.S. Surgeon General released an advisory on the subject, calling the epidemic of loneliness and isolation "an underappreciated public health crisis" on par with previous crises like smoking, substance abuse, and obesity.Holt-Lunstad, who served as the lead science editor of the new advisory, would love to see national health guidelines for social connection akin to the dietary guidelines published by the Departments of Agriculture and Health and Human Services. While dietary guidelines might steer people toward 30 grams of fiber a day, for example, social connection guidelines would specify the number of friends we should aim to interact with on a daily or weekly basis.Although any recommendations should be vetted by a committee of experts, "some of the evidence seems to suggest that we should have somewhere between four to six people in our lives and prioritize in-person over online contact," Holt-Lunstad says. And just as our diet should contain some variety, our social connections should span the range from spouse to co-worker to friend.How to stay socially connectedJoin a groupSometimes life can get hectic, so joining a group that meets regularly – whether it's a daily exercise class, a monthly book club, or a weekly knitting circle – ensures you'll see the same people consistently."When you have a regular cadence of interacting with people, it's more likely for relationships to develop, and to be sustained," Holt-Lunstad says.VolunteerBuild communities around shared purpose and values. "There is an amazing neurobiology in the brain that is all about hoping and seeking and trying to make a better world, which rewards us powerfully," Cole says. He cites a study that he and his colleagues conducted with 18 older adults who volunteered in elementary school classrooms in Los Angeles. After three and nine months in the program, participants reported feeling a sense of purpose and well-being, which was reflected in decreased inflammation and a better-balanced immune system.As a happy side effect, these communities also partially fill the void left by our vanishing neighborhoods, religious groups, and other social infrastructure. So you might just find out that fellow whale-lover is a good person to borrow a cup of sugar from, too.Reach out every dayWith some of the workforce shifted permanently to remote work, a reliable source of connection outside the home has been severed. "If you're working from home, consider joining a shared workspace a few days a week," Barbara Walker, a performance psychologist at the University of Cincinnati College of Medicine, tells Popular Mechanics. And don't forget to nurture existing relationships or reinvigorate languishing ones. Send a card or letter to distant friends or check in with a neighbor to reconnect. Communities in which there are close ties between neighbors tend to be more resilient than those that are socially disconnected—weathering natural disasters more readily, for example.Walker suggests people set a goal for contact. "Reach out to one (or more) people a day," she says. "Either via phone call or text, or better yet, in person."