How the Flu Works

Staying Healthy Image Gallery Flu season is upon us.  Get health tips with staying healthy pictures.
Staying Healthy Image Gallery Flu season is upon us.  Get health tips with staying healthy pictures.
Peter Dazeley/Photographer's Choice/Getty Images

Every winter, like clockwork, the flu returns. It inf­ects millions of ­us -- about 5 to 20 percent of the U.S. population alone, according to the Centers for Disease Control and Prevention (CDC) [source: CDC]. Health Canada estimates that 10 to 25 percent of Canadians get the flu each year [source: Health Canada]. It leaves us sniffling, sneezing, coughing, achy and generally feeling miserable for anywhere from a few days to a few weeks.

Although most of us think of the flu as a mild annoyance that we have to deal with each winter, it can actually be a very dangerous disease. The CDC estimates that in the United States alone, more than 200,000 people are hospitalized­ with the flu or with flu-related complications each year, and more than 36,000 people die from it [source: CDC]. Around the world, the flu kills between a quarter of a million to half a million people a year [source: WHO].


­In this article, we'll find out how people get the flu, what the symptoms are and how to protect yourself.

What Is the Flu?

The flu is a respiratory illness caused by the influenza virus. The flu is not the same as a cold, although they share many of the same symptoms. The cold is caused by a different virus, and it tends to have milder symptoms than the flu. Colds are also less likely to cause serious complications.

When the influenza virus gets into the body, it moves into the respiratory tract. Once there, it binds to the surface of cells. The virus then opens and releases its genetic information (RNA) into the cell's nucleus. The nucleus is where the cell's genetic information (DNA and RNA) is stored. The virus replicates, or copies itself, and takes over the functions of the cell. The copies of the virus move to the cell membrane until the cell finally dies and releases them out into the body, where they go on to infect other cells.


The respiratory tissues swell up and become inflamed (the inflammation usually heals within a few weeks). As the virus moves through the respiratory tract and into the bloodstream, the first symptoms begin to emerge. The replication process continues for up to several days, until the body's immune system begins to fight the virus off.

Flu symptoms can include any or all of the following:

  • Coughing
  • Sneezing
  • Fever
  • Body aches
  • Runny nose and congestion
  • Tiredness

These symptoms,­ although uncomfortable, are generally not dangerous. But the flu also weakens the immune system, leaving it vulnerable to more serious infections. High-risk individuals (see Who Is at Risk?) in particular are susceptible to serious complications, such as:

  • Bacterial pneumonia
  • Dehydration
  • Sinus problems and ear infections (primarily in children)
  • Worsening of preexisting conditions, such as asthma or diabetes

How Do You Get the Flu Virus?

Flu season in North America runs from November through March, but dates can vary from year to year. January and February tend to be the most active flu months.

During flu season, people begin coming down with the illness, and they quickly spread it to friends, family and coworkers. Schools are particularly notorious for spreading the flu, because students are in such close quarters. And when a child picks up the virus, he or she often brings it home and shares it with the rest of the family. The National Institute of Allergy and Infectious Diseases estimates that one out of every three families of school-aged children is infected with the flu each year.


The flu is highly contagious. It is spread primarily by coughing and sneezing (which people who have the flu tend to do a lot of). Let's say you have the flu virus. Every time you cough or sneeze, you release tiny droplets of fluid into the air. Those tiny droplets can fly pretty far -- up to 3 feet (about 1 meter). If some of those droplets land on the nose or mouth of a person standing nearby, that person is likely to get as sick as you are, usually within one to four days. You can also spread the virus if you touch something (like a doorknob or table) after you've sneezed or coughed into your hand, and then other people come along and touch the same doorknob or table and put their hand on their nose or mouth.

If you have the flu, you're not just contagious when you have symptoms. You can pass along the virus one day before you start sniffling and sneezing, and you can keep passing it along for seven days after you start sniffling and sneezing. Children can be contagious even beyond the seven days.

Who Has a Flu Risk?

Anyone can get the flu, but some groups are more susceptible than others and are at greater risk for more serious complications or even death.

Risk groups include:


  • Children under the age of 2 (whose immune system is not yet fully developed)
  • Seniors over the age of 65 (most flu deaths are among seniors)
  • Anyone who has a chronic medical condition (such as asthma or diabetes)
  • Pregnant women
  • Health care workers
  • Nursing home residents

The CDC recommends that high-risk individuals get a flu vaccination each year.

Flu Remedies

If you've got the flu, stay home, get under the covers and keep hydrated.
If you've got the flu, stay home, get under the covers and keep hydrated.
Alex and Laila/Stone/Getty Images

Unfortunately, there isn't a pill or a liquid you can take that will "cure" you of the flu. Penicillin and other antibiotics won't work, because they only kill bacteria, and the flu is caused by a virus.

There are, however, a few approved antiviral drugs, including Symmetrel, Flumadine, Relenza and Tamiflu, that have been shown to shorten the duration of the illness.


Relenza (zanamivir) and Tamiflu (oseltamivir phosphate) are neuraminidase inhibitors. They work by blocking the action of a protein called neuraminidase, which sits on the surface of a cell and normally helps the influenza virus enter and leave the cell. Neuraminidase inhibitors trap the virus once it enters a cell.

By stopping the virus from spreading to other cells, Relenza and Tamiflu lessen the severity and shorten the duration of the flu infection.

­Symmetrel and Flumadine also lessen the severity and shorten the duration of the flu, but they only work against influenza A. Both are antiviral medications that work by stopping the virus from replicating. All four drugs are by prescription only and do have potential side effects, so they should only be taken with the advice of a doctor.

The best advic­e for treating the flu is to rest and drink plenty of liquids. Over-the-counter cold and flu remedies can alleviate some of the symptoms, at least temporarily. Aspirin may relieve fever and aches, but it should not be given to children and adolescents because of the risk of a rare but potentially dangerous illness called Reye's Syndrome.

Avoiding the Flu

This guy needs to wash his hands -- and so do you.
This guy needs to wash his hands -- and so do you.
Peter Cade/Iconica/Getty Images

Experts say the best way to avoid catching the flu is to practice good hygiene during flu season. Here are a couple of tips:

  • Wash your hands throughout the day with warm water and soap.
  • Avoid anyone who is coughing or sneezing.

If you do get sick, you can avoid infecting others if you:


  • Stay home until you're feeling better.
  • Cover your mouth and nose with a tissue whenever you cough or sneeze.
  • If you have to sneeze or cough into your hands, wash them thoroughly afterward with warm water and soap.

­ Remember that you can spread the flu for up to seven days after you get sick, so be careful with your germs even after most of your symptoms have passed.

In the next section, we'll take a look at the flu vaccine and see how it wards off this illness.

Flu Vaccine

Elaine Louie receives a flu shot, offered free by the city of Chicago, from registered nurse Betty Lewis in October 2006.
Elaine Louie receives a flu shot, offered free by the city of Chicago, from registered nurse Betty Lewis in October 2006.
Tim Boyle/Getty Images

Another way to prevent the flu is by getting a vaccine at the beginning of each flu season (October or Nov­ember). The earlier you get vaccinated the better, because it takes about two weeks for the vaccine to take its full protective effect. Children under the age of 9 who have never had a flu shot especially need to get an early start, because they will need to have two vaccinations administered about one month apart.

The flu vaccine comes in two forms: a shot and a nasal spray, and works by triggering your body's immune system response. When you get a flu vaccine, your body recognizes the flu virus as a foreign invader and produces antibodies to it. The next time your body encounters the flu virus, it will remember that it is a hostile invader and quickly launch an immune attack to kill off the virus.


But if your body remembers the virus, why do you need to get a flu shot every year? First, because flu strains differ from year to year; and second, because immunity declines over time.

The shot, which is normally given in the arm, is made up of three different viruses. The three strains are chosen by scientists working in laboratories around the world. They collect flu viruses and predict which strains will be most prevalent in the coming flu season. The viruses in the shot are inactivated, or dead, which means that they can't actually give you the flu.

The nasal-spray flu vaccine is often referred to as LAIV (Live Attenuated Influenza Vaccine). Unlike the flu shot, it uses a live but weakened version of the virus. Like the shot, it contains three different flu strains. When the LAIV is sprayed into the nose, it works much like the injected form of the vaccine, stimulating the immune system to develop antibodies against the virus.

How effective is the nasal vaccine? One large study found that it reduced the incidence of flu in young children (age 1 to 7) by 92 percent. The study didn't test the effectiveness of the flu spray on adults. Because it is a live vaccine, LAIV is only recommended for healthy people between the ages of 5 and 49.

Who Should Get a Flu Shot?

Studies show anywhere from a 25 percent to a 50 percent decrease in mortality and hospitalization rates for heart patients who get the flu vaccine.
Studies show anywhere from a 25 percent to a 50 percent decrease in mortality and hospitalization rates for heart patients who get the flu vaccine.
Photo courtesy CDC


­ Anyone in a high-risk group should be vacc­inated at the beginning of every flu season. The CDC recommends that all children ages 6 to 23 months get vaccinated. Very young children are more likely to be hospitalized with the flu, and to die from it, than older children and adults. Because infants under 6 months are too young to take the vaccine safely, all people around them (family members and child care workers) should be vaccinated, as well.


Older adults (over age 65) should also be vaccinated, as well as anyone with a chronic health condition like asthma or diabetes. Plus, the CDC recommends that pregnant women and people who work in the healthcare industry be vaccinated.

Other people who shouldn't receive the flu shot are:

  • People who have had a severe reaction to the flu shot in the past
  • Anyone who developed Guillain-Barré Syndrome, a rare nerve disease, within six weeks of a previous flu shot
  • Infants under 6 months of age
  • Anyone who is running a fever
  • Anyone who is allergic to chicken eggs, because the flu vaccine is grown in eggs

Severe side effects are rare, but many people complain of mild flu-like symptoms, including low-grade fever, muscle aches and tiredness. The side effects usually begin within a few hours after getting the flu shot and can last for about two days.

To find out more about the flu, other diseases and related topics, check out the links on the next page.

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