Even before the Covid-19 coronavirus reached more than 100 countries around the world, early data from China — where the outbreak started — suggested that older adults were the most vulnerable to the worst effects of the disease.

Now, that data, along with emerging research from Italy — the second-most-affected country in the world — is showing just how dangerous Covid-19 is for older people, and others with with heart, lung, and immunological conditions.

In Italy, a country with one of the world’s oldest populations, a March 4 analysis by the national health institute found that of the 105 patients who died from the virus, the average age was 81. This put a 20-year gap between the average age of people who tested positive for the virus and the deceased, the institute said. On Friday, an ICU physician in Lombardy — the epicenter of Italy’s outbreak — told JAMA there have been only two deaths of people under the age of 50.

This finding squared with some of the other best data on the risks of the new respiratory disease, from China’s Center for Disease Control. In a late February report, researchers looked at the first 72,314 patients with confirmed or suspected Covid-19 and discovered a huge variation in the case fatality rate by age group. In short, the disease appears to be deadlier in people with each passing decade.

Age, however, doesn’t tell the whole story about who is at risk of severe disease. In fact, it reveals the underlying vulnerabilities in the wider population to an illness like Covid-19. Many of these factors are concentrated among older adults, but younger people with certain underlying health problems are also at risk. (Sixty percent of Americans have at least one chronic condition; 40 percent have more than one.)

The lessons we learn from older patients could help us treat and prevent the spread. That makes it all the more important to understand the variables that put older adults at greatest risk so we can develop a strategy to protect society as a whole.

Immune function declines with age. That makes us more susceptible to more severe illness.

As we age, the systems our bodies use to fight disease wear down. Not only does the body have a harder time fighting off new infections like Covid-19; it’s also more likely to be afflicted by chronic diseases that make the immune system weaker.

Let’s walk through the first part: how our bodies get worse at fighting foreign invaders, including viruses like SARS-CoV-2, which causes Covid-19.

In older adults, the number of white blood cells that find and help eliminate infections can decline. The cells also become less adept at identifying new pathogens to fight. In the case of Covid-19, the virus can also damage the immune cells that might otherwise overcome the virus. If there are fewer of these cells to begin with, and they’re also weaker than they once were, an illness can do more damage.

“Immune function is not as robust as it is in younger people,” said Sean Leng, a geriatrician and a professor of medicine at the Johns Hopkins University School of Medicine. “Studies over the years have shown that in most people, their immune function is pretty okay in their 60s, or even in their 70s. The immune functions go down rather quickly after age 75 or 80.”

When a response to infection kicks in, an older person’s immune system faces a higher chance of a dangerous overreaction known as a cytokine storm. Cytokines are proteins that serve as signals to the body to ramp up its infection-fighting machinery.

But during a storm, these cytokines are overproduced, which causes severe inflammation, high fever, and organ failure. In other words, it’s not just a sluggish response to infections that can harm older adults; the immune system’s overreaction to an invader can also kill, Leng said. “The cause of death of this virus is, No. 1, respiratory failure, and then No. 2, probably the cytokine storm.” The good news, as two doctors at the University of Alabama Birmingham wrote for Vox, is that we have treatments for cytokine storm syndrome, which could help save a significant number of lives in this outbreak.

Older people also have a higher prevalence of chronic disease

The longer we live, the more likely our cells are to replicate in dangerous ways, the more damage they accumulate, and the more likely our organs are to stop functioning normally. This puts us at a heightened risk of chronic health conditions, like cancer or diabetes. Along with already weakened immune systems, these underlying diseases can make it harder for the body to ward off infections. The takeaway: It’s not just age alone that endangers people; it’s being older with one or more chronic diseases.

Among the 105 patients who had died in Italy as of March 4, two-thirds had three or more preexisting conditions. The most common was hypertension, followed by ischemic heart disease and diabetes mellitus. These chronic illnesses can leave organs degraded and more vulnerable to infection. Additionally, the treatments for these conditions can suppress the immune system, leaving the body susceptible to pathogens.

In a World Health Organization report on China’s outbreak, the case fatality rate in people who reported no chronic diseases was 1.4 percent, but it shot up in groups with these conditions: “13.2% for those with cardiovascular disease, 9.2% for diabetes, 8.4% for hypertension, 8.0% for chronic respiratory disease, and 7.6% for cancer.”

Another study on China, newly published in The Lancet, found that nearly half of the 191 hospitalized patients in the sample had another underlying illness.

“These people are dying of an inflammatory process in their lungs. It’s not an infectious process, like a bacterial or viral infection. It’s inflammatory, like we see with SARS,” said Bruce Aylward, who led the WHO mission to China that produced the report. He was referring to the fact that SARS — like the new coronavirus — drove some people’s immune systems into overdrive, leading to cytokine storm.

But that immune response seems to be worse in people with underlying health conditions, the Chinese data showed. Aylward said, “the proportion of people who die who had cancer was half compared to hypertension and cardiovascular disease. Diabetes is a little bit lower than those two, and cancer lower again.”

The CDC recommends several measures to help prevent the spread of Covid-19: Wash your hands often for at least 20 seconds.

Cover your cough or sneeze with a tissue, then throw it in the trash.

Clean and disinfect frequently touched objects.

Stay home as much as possible, and do not go out if you are sick.

Wear at least a cloth mask in certain public settings.

Contact a health worker if you have symptoms. Guidance may change. Stay informed, and stay safe, with Vox’s coronavirus coverage hub.

It’s not clear why Covid-19 is especially risky for people with these diseases. But in general, respiratory diseases can be especially dangerous for people with cardiovascular disease.

When the lungs don’t function properly, the heart has to work even harder. Researchers have also known that diabetes can damage the nervous system and impair the body’s efforts to clear infections from lungs. Conditions like high blood sugar associated with diabetes can also suppress immune cells.

There are other problems associated with aging that play a role here as well. Older people may be less efficient at coughing and sneezing, making it harder for them to clear the Covid-19 virus, which infects the airways. Accumulated lung damage in older adults from habits like smoking or breathing polluted air can further increase vulnerability, so when Covid-19 strikes, it can lead to problems like severe pneumonia.

On the other hand, it’s still only a minority of older adults who are facing the most severe consequences of Covid-19. Many have recovered, and older people in otherwise good health will likely survive the infection, Leng said. “If you have a strong immune system to begin with, you can most likely fight against the virus.”

Social factors play a major role in vulnerability to disease

Beyond physiology, how society treats older adults plays a big role in their risk from diseases like Covid-19. At the same time, measures like social distancing and quarantines remain our most effective tactics for shielding the most vulnerable during this outbreak.

“Right now there isn’t a vaccine, so we have to start thinking about other ways that people can be protected,” said Julie Masters, chair of the department of gerontology at the University of Nebraska Omaha. (Geriatrics is the clinical specialty for treating the elderly; gerontology is the scientific study of aging.)

But there are trade-offs with distancing tactics, particularly for isolated older adults, as Vox’s Ezra Klein observed about a major National Academies of Sciences report looking at the health consequences of isolation:

The researchers found that even before the coronavirus, about a quarter of older adults fit the definition of socially isolated — which measures routine social contact — and 43 percent said they felt lonely. You can be socially isolated without reporting feelings of loneliness, and you can be lonely without being socially isolated. But both conditions seem to inflict harm on physical and mental health.

Conversely, researchers have found that robust social networks and interactions improve health for older adults. “We know that when people are more integrated socially, when they have people in their life, when they have a sense of meaning and purpose, that’s good for their overall well-being, their overall health,” Masters said.

That’s creating a difficult situation for older people who need protection from diseases like Covid-19 but may already be quite isolated. So the concern here isn’t just older people who may get sick; it’s people who will experience a drop in their quality of life in response to the outbreak.

Another issue is how older adults interact with the health care system. As noted earlier, age tends to bring more chronic illnesses, which in turn demand more frequent health interventions or ongoing treatment in places like nursing homes.

“With our most frail, most ill older adults, it’s not just that they’re older and they have a compromised immune system, it’s also that many times they are in situations that put them at risk, in terms of congregate living,” said Elaine Eshbaugh, a professor of gerontology at the University of Northern Iowa. “They [may] live in a nursing home where if something like [Covid-19] happens, it takes off like wildfire.”

We saw this happen at the Life Care Center in Kirkland, Washington, where Covid-19 infected 50 residents and killed 23 patients. The Centers for Disease Control and Prevention has now put out additional guidance for nursing homes and long-term care facilities to deal with the outbreak.

“Given their congregate nature and residents served (e.g., older adults often with underlying chronic medical conditions), nursing home populations are at the highest risk of being affected by COVID-19,” according to the CDC’s website. “If infected with SARS-CoV-2, the virus that causes COVID-19, residents are at increased risk of serious illness.”

But these measures to limit the spread of Covid-19 can also hamper care for other chronic illnesses among older adults, particularly as the people providing the care start to keep their distance. “As a virus like this becomes prevalent, it may not be that [older adults] get sick, it may be that the person who comes into their home to care for them every day has a cough and stays home,” Eshbaugh said.

Older adults may face the greatest risks, but the Covid-19 outbreak affects everyone

While the majority of the people infected by Covid-19 will survive, there will be significant ripple effects if large numbers of older adults get very ill or die from the disease.

That’s particularly true in the United States, where the population as a whole is getting older: There are almost 50 million Americans over the age of 65, and their ranks are growing rapidly. These people are our coworkers, employers, friends, family, caretakers, and educators. They are major contributors to our economy and our social fabric, providing value beyond what’s captured by economic metrics like GDP.

Only about 5 percent of seniors in the US live in assisted care facilities, which means the vast majority, even those with chronic illnesses, are living on their own or with family while going about their normal lives. And right now, the leading contenders for the White House are all in their 70s.

That means that everyone has to work together to control the spread of Covid-19, and we all stand to suffer if we don’t. Younger people can limit the infection’s spread to the more vulnerable with good hygiene, self-isolating when they’re sick, and practicing social distancing measures so they can avoid getting the virus and passing it along to their older family members. This is particularly important for Covid-19 since people can transmit the virus before experiencing any symptoms. And we’re still learning about the dangers of this disease and how quickly it’s spreading, so there may be other groups at risk that we don’t yet understand.

Some of the factors that put older adults at greatest risk are not unique to them. Whether you’re 75 or 35, chronic illnesses worsen outcomes from infections like the new coronavirus. Younger people have already fallen ill and have died from Covid-19, though they make up a much smaller share of the afflicted.

However, the potential damage from this virus could be immense: More than 10 million Americans are immunocompromised, and more than 100 million US adults have diabetes or prediabetes. So the measures we use to protect older adults, both physically and mentally, can help younger people in similar situations. “There are young people ... they’ve got those underlying conditions, and that’s what puts them at risk” from Covid-19, Masters said.

This outbreak is also an opportunity to rethink and strengthen our personal health and social infrastructure. “This may be a wake-up call to us to think about our own aging and what can I do to make sure I’m in the best health possible,” Masters said. “Meaning, am I getting a good diet? Am I exercising? Do I have people in my life? Do I have social connections? All those things can keep us healthy.”

But at a time when people are being forced apart — with offices closing, conferences being canceled, students staying home, gyms shutting down, and individuals isolating themselves — those social connections are becoming harder to maintain. We’ll need to make active efforts to stay in touch, for our own health, and for those around us.