UNC Health Care
A scientific image showing microscopic images of the flu virus.

The Science of the Flu: How It Makes You Miserable

The flu is simple, right? You get it from germs, it feels terrible and then you get better.

Turns out there’s a bit more going on.

Every year, millions of people will get sick from the flu—but many don’t know how it works or spreads or what they can do to prevent it.

Christopher Hurt, MD, assistant professor of infectious diseases at the UNC School of Medicine, explains the science behind the flu and how you can stay healthy this flu season.

How It Spreads

Influenza, also known as the flu, is a highly contagious respiratory infection that originates in animals such as birds and pigs. “That’s part of the reason why the surveillance for flu is really focused on what flu in animals is doing,” Dr. Hurt says, particularly in the Southern Hemisphere, where there is a high concentration of these animals in close proximity to people. (Read more about how flu predictions are made.)

Most flu transmission happens from person to person, but new strains can emerge in animals and jump to humans if they’re in close contact with infected animals. Transmission between people can happen from inhaling droplets that someone coughs or sneezes out or by picking up viruses from contaminated surfaces.

“Covering your cough—ideally by using the inside of your elbow—is a great way to help prevent the spread of infections during cold and flu season. And remember to always wash your hands before eating, too,” Dr. Hurt says.

How It Works in Your Body

The flu virus targets cells in your nose. “The virus uses special proteins to attach to those specialized respiratory cells in your nose,” says Dr. Hurt. “Once it gets inside, it hijacks those cells to start making copies of itself; it turns those cells into a Xerox machine for flu viruses.”

The process of the flu virus attaching to and taking over the cells in your nose is facilitated by a special molecule on the outside of the flu virus called hemagglutinin—that’s the “H” in the naming of influenza viruses such as H1N1 and H3N2. The other molecule, neuraminidase, is responsible for allowing newly formed flu viruses to leave the hijacked cell and spread to uninfected neighbors. The different molecular combinations are what make flu different from other viruses; it’s always changing. So not only are there several types of flu viruses, but those viruses are also always evolving.

“For example, if a pig got infected with pig flu and bird flu, then the individual gene segments of those two viruses can get swapped around. The product of that switch can be a new flu virus that nobody has ever seen before,” Dr. Hurt says, meaning no one has the right antibodies to fight it off. “That’s also part of why epidemics and pandemics can happen so quickly.”

Epidemics happen when an infectious disease spreads rapidly, and a pandemic is a global disease outbreak. Flu pandemics happen when a new flu virus arises—most recently in 2009 with H1N1, or “swine flu”—which means humans have little to no immunity against it.

Is It a Cold or Flu?

Although the common cold and the flu are both contagious viral infections, flu symptoms often come on suddenly and are much worse. You might have the flu if you are experiencing:

  • Severe aches
  • Weakness or extreme fatigue
  • Fever
  • Sore throat
  • Runny nose
  • Dry cough

“Most people think influenza is just a bad cold, but it really isn’t,” Dr. Hurt says. “A pretty common description that makes me worry about true influenza infection goes something like: ‘I was at my desk and then at like 2 o’clock, I just started to feel achy all over, got a terrible headache and then got a chill.’ That sudden onset of fever and aches suggests to me they could have real-deal influenza and not just a cold.”

Protect Yourself from the Flu

The best way to protect yourself from the flu is to get an annual flu shot.

Every year, flu experts from around the world meet in advance of flu season to determine what viruses are most prevalent and should be considered when producing the vaccine.

“It’s a bit of an educated guess, really. It’s based on what’s happening at that moment and what the other hemisphere’s flu season looked like,” Dr. Hurt says.

The chosen viruses are injected into chicken eggs, where they incubate and multiply. The eggs are then harvested, the viruses are killed and a purification process begins.

“The material that comes out of the egg has virus in it, and the vaccine manufacturers ultrapurify it and get rid of stuff that doesn’t make good antibodies. This enriches it with materials that promote stronger anti-flu antibodies,” Dr. Hurt says. Antibodies are made by cells in the immune system that kill foreign bacteria and viruses that enter the body.

“The key piece of all of this is that the flu virus is killed, so there is no live virus in the shot form of the vaccine,” Dr. Hurt says. Careful quality checks are performed throughout the manufacturing process to ensure there is no active flu in the end product.

“At the end of this process, you get vials of material that basically have purified chunks of dead virus,” he says. “Those chunks are called antigens, and they are proteins from the flu virus that have been shown to get the immune system to make antibodies.”

Flu shots are then injected into the muscle, where the body’s immune system makes antibodies in response to the antigens in the vaccine.

“If you get the flu shot, before you ever run into a flu strain during flu season, you’ve got antibodies in your body made in response to the vaccine,” Dr. Hurt says. “If and when you run into the real-deal flu, you have antibodies waiting to grab and neutralize it before it has a chance to do anything.”

Although you can’t get sick from the flu shot, it does take about two weeks for your body to develop the antibodies needed to protect you against the flu. This means it’s important to get vaccinated as early in the flu season as possible, before you come in contact with the virus. The flu vaccine is 40 to 60 percent effective, and the most common side effects are redness, soreness and swelling at the injection site.

Everyone ages 6 months and older should be vaccinated, especially people at high risk of catching flu or experiencing serious complications. That includes older adults, people with chronic lung or immune-suppressing conditions such as diabetes, pregnant women and young children.


Talk to your doctor about getting your flu shot this season. If you don’t have a doctor, find one near you.