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Why Is There A Flu Season?

Why Is The Flu Seasonal?

Camille Freking, MS Pharmacology Profile Photo

Written by Camille Freking, MS Pharmacology on December 20, 2021

After the hot days of summer, the cooling temperatures of the fall and winter months, usually October through December, are something many people look forward to. However, the falling temperatures usher in the flu season, which is not welcomed with the same enthusiasm.

The colder months are the time when people are more likely to get sick with many viruses, including influenza, but why is there a season with higher flu activity?

What Is Influenza (Flu)?

Influenza, commonly referred to as the flu, is a contagious virus that can affect the entire body. Although it is primarily a respiratory disease due to the many symptoms in the nose, throat, and lungs, the flu may also cause other symptoms.

The flu can be caused by any strain of the influenza viruses that vary slightly each year. Flu can range in severity from mild to severe, and for some people, the disease can be life-threatening.

While most people will recover completely after a few days of symptoms, immunocompromised individuals and others with certain risk factors may experience complications and more severe symptoms.

What Are the Symptoms of the Flu?

Each person experiences the flu slightly differently in terms of both severity and the types of symptoms that they experience.

Unlike the common cold, which may gradually build in intensity over the course of several days, the flu can come on suddenly. Individuals may feel symptoms within a day after being exposed and infected with the flu.

Flu symptoms include:

  • Fever or feelings of fever and chills
  • Sore throat
  • Muscle or body aches
  • Fatigue or extreme tiredness
  • Cough
  • Runny or stuffy nose
  • Headaches
  • Vomiting
  • Diarrhea

Each person experiences the flu differently, so not everyone who has the flu will experience all of the above symptoms, including fever. While many people use the presence of fever to differentiate between the flu and a cold, not all people with the flu will experience a fever. Additionally, while adults can experience vomiting and diarrhea, these symptoms are typically more common in children.

How Does the Flu Spread?

Like many infectious diseases, the flu is an airborne virus that travels from person to person through tiny droplets of moisture that carry the virus. These droplets are most commonly spread when a person who is sick with the flu speaks, coughs, or sneezes, releasing the virus into the air.

People nearby may inhale the virus into their nose or mouth while breathing, causing them to become sick. It’s also possible to catch the flu by touching a surface that has droplets of the virus on it, such as a doorknob or faucet. If you touch an infected surface and then touch your mouth, nose, or eyes, it’s possible to catch the flu.

Why Is There a Flu Season?

Although the influenza virus is present year-round, people are more likely to get sick with the flu in the colder months of the year, including fall, winter, and early spring. Theflu season describes the time period when flu activity spikes in the population. This spike occurs because the colder temperatures allow the virus to survive longer in the air.

Seasonal influenza viruses are airborne viruses that spread by traveling through the air from one person to the next; for example, it can be transmitted when a sick individual coughs or sneezes near another person.

The virus is coated by a protective gel-like coating that preserves the virus while it is airborne. This protective coating, sometimes referred to as a capsule, is made up of fats and oils that create a hard shell. The coating begins to degrade in warmer temperatures but is firmer during the colder months, which means that the virus is better able to survive in the air during the colder months.

Warm temperatures are also involved with the infection process once the virus is inside your body. Once you are infected with the virus, the warm temperatures in your body start to break down the protective capsule around it, releasing the virus into your system. Once released, the virus begins to replicate and cause symptoms.

People are typically contagious for five to seven days from the time they first begin experiencing symptoms of the flu.

Another element that influences the spike in flu infections during the colder months is that people spend more time indoors when it is cold. Unlike the warmer months, where people may congregate with friends and family outdoors, indoor gatherings, particularly around the holidays, offer an opportunity for the virus to spread quickly among a group of people.

Is It Possible To Get the Flu When It Is Not Flu Season?

Flu infections typically start to decline in the spring as the temperatures warm up, with the season tapering off significantly in May in the northern hemisphere. However, it is still possible to get infected with the flu, particularly if you live in a colder climate or spend a significant amount of time in close quarters with other people.

Additionally, the flu season varies depending on which hemisphere you are in, as winter in the northern hemisphere is summer in the southern hemisphere, and vice versa. Therefore, people traveling between the two hemispheres can catch the flu during their home’s off season, since their travel destination could be in peak flu season. In the northern hemisphere, the flu season typically peaks in February, while the southern hemisphere reaches peak flu season in August.

Who Is Most Likely To Get the Flu?

No matter what age they are, anyone can get the flu. However, some age groups are more susceptible to the flu than others.

Astudy conducted by the Center for Disease Control and Prevention (CDC) in 2018 found that children were the most likely to get sick with influenza, with approximately 9.3 percent of children becoming sick with the flu in any given year.

The likelihood of becoming sick from the flu decreases with age, with approximately 8.9 percent of adults between the ages of 18 and 64 getting sick and 3.9 percent of adults ages 65 and older becoming ill.

These percentages do not take into account the number of people who become ill but do not exhibit symptoms; rather, they reflect those who develop a symptomatic flu infection.

Although older adults are less likely to develop the flu, they may be more likely to develop complications of the illness and may be sicker for longer than children.

How Many People Get the Flu Each Year?

As noted above, the percentage of people who get the flu each year varies by age group. However, in general, the CDC estimates that approximately 8 percent of the population experiences symptomatic flu infections in the average year. This number varies depending on the severity of the current year’s strain of the flu, with a range of between 3 percent and 11 percent of the population becoming ill with symptoms.

When considering those who contract the flu but do not exhibit symptoms, the numbers are higher. An additional 2 percent to 9 percent of the population may contract the flu without exhibiting symptoms.

Overall, the total percentage of the population infected with the flu during a given year, including both symptomatic and asymptomatic individuals, ranges from 5 percent to 20 percent of the population.

How Do I Know If I Have the Flu?

The flu season typically coincides with the peak season for other common respiratory viruses due to the favorable environment created by colder temperatures. The viruses like the common cold, and respiratory syncytial virus (RSV) cause symptoms that may mimic the flu, it can be challenging to know whether you actually have the flu or if you are sick with a different type of virus, because, each person experiences the flu differently, so you may not exhibit these symptoms.

If you or a loved one starts to feel very ill during flu season or at any time of year, it’s recommended that you consult with your healthcare provider about your illness. RSV is the most common cause of severe respiratory illness in young children, including infants, and can be life-threatening in people ages 65 and older.


Flu season spans from the fall to the early spring because the protective coating surrounding the flu virus is better preserved during colder temperatures, allowing it to pass more easily from person to person.

Symptoms of the flu commonly include fever, muscle or body aches, stuffy or runny nose, fatigue, sore throat, headache, and more. Flu shots can help prevent illness and boost immunity, and may be especially important for those at a high risk of flu-related complications.