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When Is Flu Season?

Understanding The Flu

Camille Freking, MS Pharmacology Profile Photo

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

The dropping temperatures are often a welcome change from the heat and humidity of summer, but an increased risk of catching the flu is also associated with colder weather. The flu virus can cause symptoms ranging from mild to severe and can be life-threatening for some people, but when is flu season and how can you minimize your risk?

What Is Influenza (Flu)?

The influenza virus, better known as the flu, is a contagious respiratory illness that causes a severe viral infection that causes symptoms throughout the body. While the flu is primarily considered a respiratory infection, symptoms of the illness also include gastrointestinal symptoms, headache, fever, and more. The flu affects each person differently, with some individuals becoming very sick and others experiencing more mild illness.

There are three types of the flu that affect humans: influenza A, B, and C. Each of the types of influenza is caused by many different strains of the virus that mutate slightly from year to year.

Influenza A is the most common form of the illness and is the source of flu pandemics that spread across the globe. Influenza B is another severe form of the flu that may be less contagious and tends to spread locally; it is not usually associated with global pandemics. Influenza C is the most mild form of the virus; it may cause mild respiratory symptoms in some people while others experience no symptoms at all.

The flu virus itself consists of a core of genetic material such as mRNA, RNA, or DNA and a protective coating of protein that surrounds the genetic material. The protective protein coating is broken down by the warm temperatures inside the body, releasing the genetic material into the body. From there, the virus begins to replicate, causing a viral infection.

Influenza viruses can cause a wide range of symptoms that can cause life-threatening flu-related complications. Individuals who are very young, very old, immunocompromised, or suffering from chronic health conditions, like heart disease or asthma, are considered more likely to experience complications.

What Are the Symptoms of the Flu?

Although the flu is primarily considered a respiratory illness, it can cause symptoms throughout the body. Because the disease affects everyone differently, some people experience mild symptoms while others may become severely ill.

Flu symptoms typically develop quickly, with some people experiencing symptoms within a day of contracting the virus. This is one of the most significant differentiators between the flu and the common cold, which typically builds in intensity over the course of several days.

Flu symptoms include:

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

The flu is expressed differently in everyone, with symptoms varying from person to person. As a result, it is possible for two people in the same house who are sick with the flu to experience different symptoms.

Although a fever is one of the most well-known symptoms of the flu and is often used to distinguish between influenza and the common cold, not everyone who comes down with the flu will have a fever.

Additionally, although it is possible for anyone to experience any of the symptoms listed above, children are considered more likely to experience gastrointestinal symptoms like diarrhea, nausea, and vomiting than adults.

How Does the Flu Spread From Person To Person?

Flu spreads from person to person in two different ways: through the air and through contact on shared surfaces. The flu is primarily an airborne virus that is transmitted from person to person when an infected individual sneezes, speaks, or coughs.

The water vapor that is released by people who are sick with the flu contains the virus, and this water vapor can be inhaled by anyone in the vicinity of the infected person. Once the virus is inhaled, the protective protein coating that surrounds the genetic material dissolves and the virus starts replicating, causing a viral infection.

The flu can also be transmitted through contact with shared surfaces like doorknobs, tablets, or cell phones. If a person who is sick with the flu has remnants of the virus on their hands or body and they touch a surface, the virus can live on the surface for a short period of time.

A healthy individual who touches the surface and then touches their mouth, eyes, or nose can become sick with the virus. However, this method of transmission is less common than the airborne transmission of the virus. Flu season occurs when the number of people infected spikes during the year.

When Is Flu Season?

The flu season occurs during the colder months of the year, like December, in the northern hemisphere. Colder temperatures are more hospitable to the virus because they minimize the degradation of the protein coating that surrounds the genetic material at the virus’ center.

In the northern hemisphere, the influenza season typically runs from October through April or May depending on the location, while the southern hemisphere experiences the flu season from April through October or November.

Colder temperatures typically cause people to spend more time congregating indoors, which also increases transmission rates of the flu. Holiday gatherings that occur during the colder months present a significant transmission risk as well.

As the temperatures warm up, flu activity starts to subside. However, it is possible to get the flu any time of year, particularly if you spend a lot of time indoors or in close quarters. People are typically contagious and able to spread flu viruses beginning 24 hours before the onset of symptoms and for up to five to seven days after symptoms begin.

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

Although you are more likely to contract the flu during flu season, it is possible to get sick with the flu no matter the time of year or temperature. People who spend a significant amount of time indoors, live in a colder climate, or are regularly in close proximity to others are considered to be at increased risk of getting sick.

The timing of the flu season depends on where you live. In the northern hemisphere, flu season typically occurs between October and April. However, if you travel to the southern hemisphere in August, you’d be traveling during the peak of their flu season. Therefore, it’s especially important to take prevention measures when traveling.

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. The flu vaccine gives you immunity against illness, and is available at most doctor’s offices and pharmacies. Getting your flu shot each year can help you stay healthy.

Who Is Most Likely To Get Sick With the Flu?

The onset of flu season increases everyone’s risk of getting sick with the virus no matter their age or location. While it is possible for anyone to get the flu, some groups are considered to be more at risk of getting sick than others, like older people or people with diabetes.

The Centers for Disease Control and Prevention (CDC) concluded that the age group most likely to contract the flu is children under the age of 18.

A study conducted in 2018 showed that an estimated 9.3 percent of children come down with the flu in any given year, with the risk of getting sick decreasing with age. An estimated 8.9 percent of adults between the ages of 18 and 64 get sick with the flu each year, while approximately 3.9 percent of adults over the age of 65 get sick. Although older adults are less likely to get sick with the flu than other age groups, they are considered to be at increased risk of experiencing complications from the virus.

In addition to the percentages listed in the CDC study, many more people contract the flu each year without developing symptoms. While these people never actually feel sick, they are still contagious and are capable of spreading the virus to people around them. Asymptomatic individuals are responsible for much of the spread of the flu each year.


The flu season begins in the fall and ends in the spring, peaking during the coldest months of the year. For people in the northern hemisphere, the average flu season begins in October and runs through April or May, peaking in February. In the southern hemisphere, flu season begins in April and lasts through October or November, peaking in August.