Here are the seven reasons the coronavirus causing COVID-19 hits some people harder than others, why it kills some and spares others :

Though scientists at first thought age was the dominant factor, with young people avoiding the worst outcomes, new research has revealed a suite of features impacting disease severity.

These influences could explain why some perfectly healthy 20-year-old with the disease is in dire straits, while an older 70-year-old dodges the need for critical interventions.

Underlying health conditions are thought to be an important factor influencing disease severity.

Indeed, a study of more than 1.3 million COVID-19 cases in the United States, published June 15 in the journal Morbidity and Mortality Weekly Report, found that rates of hospitalisations were six times higher and rates of death were 12 times higher among COVID-19 patients with underlying conditions, compared with patients without underlying conditions.

The most commonly reported underlying conditions were heart disease, diabetes and chronic lung disease.

In general, risk factors for more severe COVID-19 outcomes include: Age, Diabetes (type 1 and type 2), Heart disease and hypertension and Smoking.

The other risk factors are Blood type, Obesity and Genetic factors.

Age

About 8 out of 10 deaths associated with COVID-19 in the U.S. have occurred in adults ages 65 and older, according to the U.S. Centers for Disease Control and Prevention (CDC). The risk of dying from the infection, and the likelihood of requiring hospitalization or intensive medical care, increases significantly with age. For instance, adults ages 65-84 make up an estimated 4-11% of COVID-19 deaths in the U.S, while adults ages 85 and above make up 10-27%.

The trend may be due, in part, to the fact that many elderly people have chronic medical conditions, such as heart disease and diabetes, that can exacerbate the symptoms of COVID-19, according to the CDC. The ability of the immune system to fight off pathogens also declines with age, leaving elderly people vulnerable to severe viral infections, Stat News reported.

Diabetes

Diabetes mellitus — a group of diseases that result in harmful high blood sugar levels — also seems to be linked to risk of more severe COVID-19 infections.

The most common form in the U.S. is type 2 diabetes, which occurs when the body’s cells don’t respond to the hormone insulin. As a result, the sugar that would otherwise move from the bloodstream into cells to be used as energy just builds up in the bloodstream. (When the pancreas makes little to no insulin in the first place, the condition is called type 1 diabetes.)

In a review of 13 relevant studies, scientists found that people with diabetes were nearly 3.7 times more likely to have a critical case of COVID-19 or to die from the disease compared with COVID-19 patients without any underlying health conditions (including diabetes, hypertension, heart disease or respiratory disease), they reported online April 23 in the Journal of Infection.

Even so, scientists don’t know whether diabetes is directly increasing severity or whether other health conditions that seem to tag along with diabetes, including cardiovascular and kidney conditions, are to blame.

Heart disease and hypertension

People with conditions that affect the cardiovascular system, such as heart disease and hypertension, generally suffer worse complications from COVID-19 than those with no preexisting conditions, according to the American Heart Association. That said, historically healthy people can also suffer heart damage from the viral infection.

The first reported coronavirus death in the U.S., for instance, occurred when the virus somehow damaged a woman’s heart muscle, eventually causing it to burst, Live Science reported.

The 57-year-old maintained good health and exercised regularly before becoming infected, and she reportedly had a healthy heart of “normal size and weight.”

A study of COVID-19 patients in Wuhan, China, found that more than 1 in 5 patients developed heart damage — some of the sampled patients had existing heart conditions, and some did not.

In seeing these patterns emerge, scientists developed several theories as to why COVID-19 might hurt both damaged hearts and healthy ones, according to a Live Science report.

In one scenario, by attacking the lungs directly, the virus might deplete the body’s supply of oxygen to the point that the heart must work harder to pump oxygenated blood through the body.

The virus might also attack the heart directly, as cardiac tissue contains angiotensin-converting enzyme 2 (ACE2) — a molecule that the virus plugs into to infect cells.

In some individuals, COVID-19 can also kickstart an overblown immune response known as a cytokine storm, wherein the body becomes severely inflamed and the heart could suffer damage as a result.

Smoking

People who smoke cigarettes may be prone to severe COVID-19 infections, meaning they face a heightened risk of developing pneumonia, suffering organ damage and requiring breathing support.

A study of more than 1,000 patients in China, published in the New England Journal of Medicine, illustrates this trend: 12.3% of current smokers included in the study were admitted to an ICU, were placed on a ventilator or died, as compared with 4.7% of nonsmokers.

Cigarette smoke might render the body vulnerable to the coronavirus in several ways, according to a recent Live Science report.

At baseline, smokers may be vulnerable to catching viral infections because smoke exposure dampens the immune system over time, damages tissues of the respiratory tract and triggers chronic inflammation. Smoking is also associated with a multitude of medical conditions, such as emphysema and atherosclerosis, which could exacerbate the symptoms of COVID-19.

A recent study, posted March 31 to the preprint database bioRxiv, proposed a more speculative explanation as to why COVID-19 hits smokers harder.

The preliminary research has not yet been peer-reviewed, but early interpretations of the data suggest that smoke exposure increases the number of ACE2 receptors in the lungs — the receptor that SARS-CoV-2 plugs into to infect cells.

Many of the receptors appear on so-called goblet and club cells, which secrete a mucus-like fluid to protect respiratory tissues from pathogens, debris and toxins.

It’s well-established that these cells grow in number the longer a person smokes, but scientists don’t know whether the subsequent boost in ACE2 receptors directly translates to worse COVID-19 symptoms.

What’s more, it’s unknown whether high ACE2 levels are relatively unique to smokers, or common among people with chronic lung conditions.

Obesity

Several early studies have suggested a link between obesity and more severe COVID-19 disease in people.

One study, which analyzed a group of COVID-19 patients who were younger than the age of 60 in New York City, found that those who were obese were twice as likely as non-obese individuals to be hospitalised and were 1.8 times as likely to be admitted into critical care.

“This has important and practical implications” in a country like the U.S. where nearly 40% of adults are obese, the authors wrote in the study, which was accepted into the journal Clinical Infectious Diseases but not yet peer-reviewed or published.

Similarly, another preliminary study that hasn’t yet been peer-reviewed found that the two biggest risk factors for being hospitalised from the coronavirus are age and obesity.

This study, published in medRxiv looked at data from thousands of COVID-19 patients in New York City, but studies from other cities around the world found similar results, as reported by The New York Times.

A preliminary study from Shenzhen, China, which also hasn’t been peer-reviewed, found that obese COVID-19 patients were more than twice as likely to develop severe pneumonia as compared with patients who were normal weight, according to the report published as a preprint online in the journal The Lancet Infectious Diseases.

Those who were overweight, but not obese, had an 86% higher risk of developing severe pneumonia than did people of “normal” weight, the authors reported.

Another study, accepted into the journal Obesity and peer-reviewed, found that nearly half of 124 COVID-19 patients admitted to an intensive care unit in Lille, France, were obese.

It’s not clear why obesity is linked to more hospitalisations and more severe COVID-19 disease, but there are several possibilities, the authors wrote in the study.

Obesity is generally thought of as a risk factor for severe infection. For example, those who are obese had longer and more severe disease during the swine flu epidemic, the authors wrote.

Obese patients might also have reduced lung capacity or increased inflammation in the body. A greater number of inflammatory molecules circulating in the body might cause harmful immune responses and lead to severe disease.

Blood type

Blood type seems to be a predictor of how susceptible a person is to contracting SARS-CoV-2, though scientists haven’t found a link between blood type per se and severity of disease.

Jiao Zhao, of The Southern University of Science and Technology, Shenzhen, and colleagues looked at blood types of 2,173 patients with COVID-19 in three hospitals in Wuhan, China, as well as blood types of more than 23,000 non-COVID-19 individuals in Wuhan and Shenzhen.

They found that individuals with blood types in the A group (A-positive, A-negative and AB-positive, AB-negative) were at a higher risk of contracting the disease compared with non-A-group types.

People with O blood types (O-negative and O-positive) had a lower risk of getting the infection compared with non-O blood types, the scientists wrote in the preprint database medRxiv on March 27; the study has yet to be reviewed by peers in the field.

In a more recent study of blood type and COVID-19, published online April 11 to medRxiv, scientists looked at 1,559 people tested for SARS-CoV-2 at New York Presbyterian hospital; of those, 682 tested positive.

Individuals with A blood types (A-positive and A-negative) were 33% more likely to test positive than other blood types and both O-negative and O-positive blood types were less likely to test positive than other blood groups.

There’s a 95% chance that the increase in risk ranges from 7% to 67% more likely.) Though only 68 individuals with an AB blood type were included, the results showed this group was also less likely than others to test positive for COVID-19.

The researchers considered associations between blood type and risk factors for COVID-19, including age, sex, whether a person was overweight, other underlying health conditions such as diabetes mellitus, hypertension, pulmonary diseases and cardiovascular diseases.

Some of these factors are linked to blood type, they found, with a link between diabetes and B and A-negative blood types, between overweight status and O-positive blood groups, for instance, among others.

Genetic factors

Many medical conditions can worsen the symptoms of COVID-19, but why do historically healthy people sometimes fall dangerously ill or die from the virus? Scientists suspect that certain genetic factors may leave some people especially susceptible to the disease, and many research groups aim to pinpoint exactly where those vulnerabilities lie in our genetic code.

In one scenario, the genes that instruct cells to build ACE2 receptors may differ between people who contract severe infections and those who hardly develop any symptoms at all, Science magazine reported. Alternatively, differences may lie in genes that help rally the immune system against invasive pathogens, according to a recent Live Science report.

For instance, a study published April 17 in the Journal of Virology suggests that specific combinations of human leukocyte antigen (HLA) genes, which train immune cells to recognize germs, may be protective against SARS-CoV-2, while other combinations leave the body open to attack.

HLAs represent just one cog in our immune system machinery, though, so their relative influence over COVID-19 infection remains unclear.

Additionally, the Journal of Virology study only used computer models to simulate HLA activity against the coronavirus; clinical and genetic data from COVID-19 patients would be needed to flesh out the role of HLAs in real-life immune responses.