This Is Your Body on Flu

McKenna Princing Fact Checked
A woman sitting on her couch wrapped in a blanket blows her nose.
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Along with colorful leaves, cozy sweaters, pumpkin spice lattes and all the things that make fall great is something predictable yet decidedly less fun: the seasonal flu. 

You know that influenza hits in the autumn and that it sucks. But how much do you actually know about the virus itself?

Understanding how the virus infects you — and how the vaccine protects you — will make you better prepared to handle flu season this year.

A note about flu season

Flu usually starts making the news in October, when pharmacies and doctors’ offices start encouraging everyone to get vaccinated. Though flu season usually occurs between autumn and winter, it sometimes lingers into spring. 

“Last year we had a very late, very long flu season that peaked in May, which is highly unusual, all across the U.S. The reason for that was because there were two peaks of flu,” says Dr. Helen Chu, an infectious disease specialist at UW Medicine who studies the flu.

Typically, the Seattle area sees flu season hit hardest in January or February, she says.

Flu is (and isn’t) easy to get

Though we tend to think of the flu as a single virus, it actually has four different types: flu A, B, C and D. Flu C is relatively mild and flu D doesn’t infect humans, but flus A and B include the seasonal viruses that ramp up each autumn, plus others (like H1N1, the infamous swine flu). 

According to estimates from the Centers for Disease Control and Prevention (CDC), millions — if not tens of millions — of people in the United States get the flu each year. That might make it seem like flu is an extremely contagious virus to have, but that isn’t the case, Chu says.

Virus potency is indicated by scientists using a standard marker called “r nought,” represented by Rø. A virus’s Rø number corresponds to how many people an infected person is likely to spread the disease to. 

Flu, perhaps surprisingly, only has an Rø of 1.5 — meaning that one person infected with flu will infect less than two others, on average. In comparison, the virus with the highest potency is measles, which has an Rø of between 12 and 18. 

How flu infiltrates your body

The flu virus typically enters your body through your nose via droplets from an infected person who sneezes or coughs near you. If a sick person is standing within six feet of you, they’re close enough to spread germs.  

Once in your nose, the virus sets up residence, infecting the cells in your nasal passageways and airways. The virus enters a cell and replicates, making daughter viruses (aka copies of itself) that then go and infect more cells nearby. This continues until more and more of your cells are infected. 

Most people who get the flu feel sick for three to five days, but you can feel crummy for longer, Chu says. You can infect someone else a day before your symptoms appear and up to seven days after. You’re most contagious within the first three to four days after you notice symptoms.

Once inside your cells, the virus is able to remain undetected by your immune system, essentially hiding in plain sight — for a little while, at least.

What actually causes your flu symptoms

When you catch the flu, your body becomes a battleground. 

The flu virus is an invader your body wants to oust. A microscopic battle ensues. While the virus enters your cells and replicates, your cells go on the defense, releasing chemical signals called cytokines that call for aid from your immune cells. 

It’s these cytokines, not the flu, that makes you feel so crummy. They cause the fever, muscle aches and other flu symptoms that most of us typically associate with the virus itself but are actually a result of inflammation as your body goes on the defense.

But the scene gets even more dramatic. Infected cells will display bits of the virus on their surfaces to signal to your immune cells that they’re compromised. Sometimes, infected cells even sacrifice themselves for the greater good, a process known as apoptosis.  

What happens when the virus isn’t killed

Though the presence of the flu virus will activate your body’s natural defenses, they won’t be able to eradicate the virus immediately. It takes your body two weeks to mount a strong immune response to any invaders, Chu says. 

A lot can happen in two weeks. 

If the virus spreads into your lower airways and lungs, it can cause pneumonia. In rare cases, people can even get bacterial pneumonia as a result of the flu, but this isn’t common and is more likely to happen in people who already have a medical condition or who are very young, very old or are pregnant.

Other things can go wrong, too. Inflammation is your body’s best natural defense against the flu, but too much inflammation can lead to even bigger problems, like a heart attack, damage to the heart directly, or even encephalitis.

These kinds of complications are what often lead to people dying from the flu, Chu says. During the 2017-2018 flu season, 186 children and nearly 80,000 adults died, according to CDC estimates.  

Even though your body is equipped to attack the virus on its own, it isn’t always a good idea to leave it to its own devices. That’s where the flu vaccine comes in.

How the flu vaccine protects you

Some people say getting their flu shot makes them feel like they have the flu. There’s some truth to this, though not in the way you might think.

Vaccines work by introducing a small, harmless fragments of the flu virus or inactive versions of a virus to your body. The flu vaccine is no exception. 

But getting exposed to the virus this way is way safer than actually getting infected. Why? It all comes down to antibodies. 

Exposure to the virus trains your immune cells to produce antibodies that will bind to the virus, neutralizing it and marking it for destruction. If you get exposed to the full virus later, rather than just the bits in the vaccine, your antibodies will be ready. 

Sure, the vaccine isn’t perfect. You’ve probably heard how some years it’s less effective than other years, since scientists can’t always predict which strain of the seasonal flu will be most potent any given year. And yes, because of this imperfect process, it’s still possible to get infected with the flu even after you get the vaccine. 

However, due to the way the vaccine gets your antibody response ready to tackle any future threats, getting the vaccine is still the best thing to do to protect yourself from the flu. 

Even if this year’s vaccine isn’t super effective against seasonal flu, it will still protect you against other types of flu. And since the seasonal flu virus mutates on the regular, it’s important to get a vaccine each year so you know you have at least some protection against new strains of the virus.

“Three of the four types of flu don’t change much each year. It’s just the fourth one, the seasonal flu, that we have trouble predicting,” Chu explains.

And yes, if you’re healthy and like to regale your coworkers with tales of how you never get the flu, you still need to get the shot.

“A lot of the time, healthy people aren’t going to get that sick from the flu, but what they are going to do is transmit it to other people who are going to get very sick. It’s an important concept that needs to be reinforced more,” she says.