Why does the flu make some people sick but not others?

You’ve likely noticed that some people get the flu while others don’t. Maybe you know someone who takes preventive measures to fend off the flu but still gets it, while others seem to go season after season without experiencing even a slight cough. Could their immune system have a unique ability to fight off the virus? Here, doctors offer some insight.

Are some people less susceptible to the flu than others?

The answer is ... maybe. “This is a great question that applies to any infectious disease beyond influenza,” Dr. Jonathan D. Grein, director of hospital epidemiology at Cedars-Sinai, tells Yahoo Life. “The short answer is we don't usually know why some people get sicker than others. The longer answer is there are many complex factors that go into how the immune system responds to an infection.”

For starters, those who are immunocompromised due to a preexisting health condition have an increased risk of falling ill, he explains. A study published in the journal the Lancet discovered that survivors of certain cancer types — including leukemia, lymphoma and multiple myeloma — are more likely to have serious complications from the flu up to 10 years after receiving their diagnosis.

Age plays a role as well. Adults 65 years of age and older can have a weaker immune response to flu vaccines, making them more likely to get sick with the flu or get flu complications even when vaccinated, according to the CDC. Flu complications include bacterial pneumonia, ear infections and sinus infections and can make other health conditions, such as diabetes, asthma and congestive heart failure, worse.

Lifestyle can also have an impact on one’s immunity. “For example, smoking or excessive alcohol consumption can weaken the immune system, making an individual more susceptible to infections like the flu,” Dr. John Whyte, chief medical officer of WebMD and a former director at the U.S. Food and Drug Administration, tells Yahoo Life. “Plus, high levels of stress, lack of adequate sleep and a diet lacking in essential nutrients can compromise the body’s natural defenses, making one more prone to catching the flu and having a harder time fighting it.”

He adds that exposure to the virus is yet another factor. “People who are frequently in close contact with others, such as those working in schools or health care settings, may have a higher exposure to flu viruses and may develop some immunity,” explains Whyte.

Genetics play a significant role

Whyte says that genetics also affect how a person’s immune system responds to the flu virus. “Some individuals may have genetic variations that make their immune systems less effective at fighting off influenza,” he explains.

A new study published in the journal Science found differences in how diverse populations respond to influenza. After collecting cells from men of European and African genetic ancestry and exposing those cells to the flu virus, the investigators determined that the cells of those with European ancestry displayed an increase in the activity of type I interferons — proteins that are critical for fighting viral infections — during the beginning phase of infection.

“Inducing a strong type I interferon pathway response early upon infection stops the virus from replicating and may therefore have a direct impact on the body’s ability to control the virus,” said Luis Barreiro, senior study author and associate professor of medicine at the University of Chicago, in a press release. This could also help explain why influenza outcomes can differ between racial groups, according to the researchers, who note that Black men and women have higher hospitalization rates for the flu.

But this isn’t the only study that has analyzed the flu’s interaction with the immune system. Biologists at Fred Hutchinson Cancer Research Center discovered that a single mutation in a flu virus can give it the power to escape 90% of one person’s antibody immunity, but not another’s.

Dr. Linda Yancey, an infectious diseases specialist at Memorial Hermann hospital, tells Yahoo Life: “Everyone's immune system is unique to them alone and is as different from anyone else’s as our genome,” which is the entire set of DNA instructions located in a cell. “Different people will have different immune responses to different pathogens,” she says. “There is no such thing as an identical immune response across a population.”

Can you have the flu and not have symptoms?

Keep in mind that it’s possible to be infected with influenza and have minimal to no symptoms, notes Grein. “It is generally accepted that about 20% to 30% of people will be asymptomatic or with minimal symptoms, but other studies have shown that that number is much higher, anywhere from 50% to 75%,” he says.

And yes, someone who does not feel sick with the flu could be contagious. “We know that people infected with influenza tend to begin excreting the virus from the respiratory tract one to two days before their symptoms begin,” says Grein. “So we can extrapolate that asymptomatic people are probably infectious. But it’s not well defined exactly how infectious they are.”

This is one of the main reasons why all health care workers are required to get the flu shot every fall, says Yancey. “We do not want to pass along a virus that we don't know we have to our immunocompromised patients.”

Annual flu shots help

Regardless of how strong you might think your immune system is, all three physicians strongly encourage the annual flu shot. “Even when the flu vaccine is not 100% protective — and it usually isn’t — it still can make infection less serious,” says Whyte.

Yancey estimates that about five patients from her practice die each year from influenza pneumonia. “These patients tend to be males in their 30s, 40s and 50s,” she says. “It doesn’t occur to a lot of working-age men that they need a flu shot, which leaves this population vulnerable to severe disease and death. If not for you, then please get the flu shot for your kids.”

Influenza predictably circulates every year and predictably causes severe illness and death in tens of thousands of Americans every year, says Grein. “The argument of, ‘Well, I never get sick from the flu, so I don't need the vaccine,’ sounds a bit like someone saying, ‘I've never been in a car accident so why should I wear a seat belt?’” he says. “Flu vaccines are one the most widely studied and safest vaccines we have and are the best tool to protect us from the most severe complications of the virus.”

And as far as why some people seem to have the power to evade the flu, Grein says there is no definitive answer — yet. “There are a lot of variables that go into the complexity of the immune response, which is why there remain a lot of questions.”