Understanding the Flu: What It Does and How It Spreads

Each winter, like clockwork, the flu makes its return, infecting millions. In the U.S., 5 to 20 percent of the population is affected, according to the CDC. Health Canada reports that 10 to 25 percent of Canadians also get the flu annually. It causes symptoms like sneezing, coughing, fever, body aches, and fatigue, lasting from a few days to several weeks.

Although many view the flu as a mere seasonal inconvenience, it can be far more severe. The CDC reports over 200,000 hospitalizations in the U.S. due to the flu each year, with more than 36,000 fatalities. Globally, the flu claims between 250,000 and 500,000 lives annually, as per WHO data.

In this article, we will explore how the flu spreads, what its symptoms are, and the best ways to protect yourself from it.

What Is the Flu?

The flu is a respiratory illness caused by the influenza virus. It is not the same as a cold, although both share several symptoms. Unlike the flu, a cold is caused by a different virus and typically produces milder symptoms. Colds are also less likely to cause severe health issues.

When the influenza virus enters the body, it travels to the respiratory system, where it attaches to cells. The virus then opens up, releasing its RNA into the cell’s nucleus. This is where the cell’s DNA and RNA are stored. The virus replicates within the cell, taking control of its functions. New copies of the virus move to the cell's surface, causing the cell to die and release them to infect other cells.

As the virus spreads through the respiratory system and enters the bloodstream, inflammation occurs in the tissues, which typically heals within a few weeks. As the replication process continues for several days, the body's immune system starts to fight off the virus.

Flu symptoms may include some or all of the following:

While these symptoms can be uncomfortable, they are typically not life-threatening. However, the flu can weaken the immune system, making it more vulnerable to other serious infections. People at higher risk (see Who Is at Risk?) are especially prone to severe complications, such as:

How Does One Contract the Flu Virus?

In North America, flu season typically spans from November to March, though the exact timing can shift each year. The peak of the flu season tends to fall in January and February, when flu activity is highest.

Throughout flu season, individuals start to experience symptoms of the illness and swiftly transmit it to others, including friends, family, and coworkers. Schools are especially known for being hotspots for flu transmission, as students are in close proximity to one another. When a child contracts the virus, they often bring it home, affecting the entire household. According to the National Institute of Allergy and Infectious Diseases, roughly one-third of families with school-aged children face flu infections annually.

The flu is extremely contagious. It spreads mainly through coughing and sneezing, which people with the flu do frequently. Imagine having the flu virus—every time you cough or sneeze, tiny droplets of fluid are released into the air. These droplets can travel quite a distance, up to 3 feet (around 1 meter). If any droplets land on someone's nose or mouth nearby, they're likely to become infected, typically within one to four days. The virus can also be spread when you touch surfaces like doorknobs or tables after coughing or sneezing into your hand, and others may pick it up by touching the same surfaces and then touching their face.

Even if you have the flu, you're not only contagious once symptoms appear. You can begin spreading the virus one day before showing any signs, and you can continue to transmit it for up to seven days after symptoms start. Children, however, may remain contagious for even longer than seven days.

Who Is at Risk of Getting the Flu?

While anyone can contract the flu, certain groups are at higher risk and are more likely to face severe complications or even death as a result.

High-risk groups include:

The CDC advises that individuals who are at high risk should receive a flu vaccine every year.

Treating the Flu

There isn't a simple pill or liquid that can completely 'cure' the flu. Antibiotics like penicillin are ineffective because they target bacteria, whereas the flu is caused by a virus.

However, a number of approved antiviral medications, such as Symmetrel, Flumadine, Relenza, and Tamiflu, have been shown to help reduce the duration of flu symptoms.

Relenza (zanamivir) and Tamiflu (oseltamivir phosphate) are both neuraminidase inhibitors. These drugs work by blocking neuraminidase, a protein that assists the flu virus in entering and exiting cells. By doing so, they trap the virus after it has entered a cell.

By preventing the virus from spreading to neighboring cells, Relenza and Tamiflu reduce the severity and shorten the length of the flu infection.

Symmetrel and Flumadine also help lessen the severity and duration of the flu, but they are effective only against influenza A. These antiviral drugs work by halting the virus's replication. All four medications require a prescription and may have side effects, so it's important to consult a doctor before taking them.

The best way to treat the flu is to rest and stay hydrated by drinking plenty of fluids. Over-the-counter medications for cold and flu can help alleviate symptoms, though temporarily. Aspirin can reduce fever and body aches, but should not be given to children or adolescents due to the risk of Reye's Syndrome, a rare but serious illness.

Avoiding the Flu

Experts recommend that the best way to prevent the flu is by practicing good hygiene, especially during flu season. Here are a few tips:

If you do become ill, you can help prevent spreading the infection to others by:

Keep in mind that you can still spread the flu for up to seven days after becoming ill, so it's important to be mindful of your germs even after most of your symptoms have subsided.

Next, we'll explore the flu vaccine and how it helps prevent this illness.

Flu Vaccine

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.

If your body remembers the virus, why is it necessary to get a flu shot every year? The reason is twofold: first, flu strains change each year, and second, immunity decreases over time.

The flu shot, typically administered in the arm, consists of three different virus strains. These strains are selected by scientists globally, who collect and analyze flu viruses to predict which ones will be most common during the upcoming flu season. The viruses in the vaccine are inactivated, meaning they are 'dead' and cannot cause the flu.

The nasal-spray flu vaccine, commonly known as LAIV (Live Attenuated Influenza Vaccine), differs from the flu shot in that it contains a live but weakened form of the virus. Like the shot, it contains three different strains of the flu virus. When the spray is administered through the nose, it stimulates the immune system to produce antibodies to fight the virus, similar to the injected vaccine.

How effective is the nasal vaccine? A large study found that it reduced flu cases by 92 percent in young children (ages 1 to 7). However, the study did not assess its effectiveness in adults. Because LAIV is a live vaccine, it is recommended only for healthy individuals between the ages of 5 and 49.

Who Should Receive a Flu Vaccine?

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People in high-risk categories should receive the flu vaccine at the start of every flu season. The CDC recommends that all children aged 6 to 23 months get vaccinated, as they are more likely to be hospitalized or even die from the flu compared to older children and adults. Since infants under 6 months cannot safely receive the vaccine, it is important that everyone around them (such as family members and childcare providers) get vaccinated.

Vaccination is also recommended for older adults (ages 65 and above), and for anyone with a chronic health condition like asthma or diabetes. Additionally, the CDC advises that pregnant women and healthcare workers receive the flu vaccine.

Here are the individuals who should avoid getting the flu shot:

Though severe side effects are uncommon, some individuals may experience mild flu-like symptoms, such as a low-grade fever, muscle aches, and fatigue. These symptoms typically start a few hours after the vaccination and can last for about two days.