Flu season

Flu season

Flu season is a time each year when many people get sick with the flu. The flu is caused by a virus called influenza. This usually happens during the colder months, depending on whether you live in the northern or southern hemisphere. After someone gets the flu, it often takes about two days before they start feeling sick.

Doctors can sometimes guess when and where flu outbreaks will happen and track them as they spread. In any one place, small outbreaks of the flu usually grow bigger over about three weeks and then get smaller again over another three weeks.

It’s important to stay healthy during flu season and get a flu shot if you can.

Cause

Main article: Influenza

Three types of viruses, Influenza virus A, B, and C, are the main reasons people get the flu. During cooler weather, cases of the flu go up a lot. Even though we hear more about the flu during certain times of the year, these viruses can spread all year long.

Each year, the flu is usually caused by a different type of virus. This happens because people can become resistant to the virus from past infections or vaccines, and the viruses also change over time. We are not exactly sure why the flu shows up more in certain seasons, but there are some ideas. People spend more time indoors during winter, which can help the virus spread. Less sunlight might also make it harder for our bodies to fight off infections. Cold, dry air can dry out the membranes in our noses and throats, making it easier for viruses to get in. Vitamin D, which our bodies make with sunlight, can also affect our immune system and might change with the seasons.

Timing

The flu season in the United States is from October to May, usually getting strongest in February. In Australia, it runs from May to October and often peaks in August. Other countries in the southern half of the world, like Argentina, Chile, South Africa, and Paraguay, also see their flu season start around June.

Flu seasons can happen even in warm places, though they change by location. For example, in Hong Kong, the flu season is from December to March, during the cooler months of winter and early spring.

Flu vaccinations

Main article: Influenza vaccine

Flu vaccinations help make flu season less serious and can cut an individual's risk of getting the flu in half. Because the Northern and Southern Hemisphere have winter at different times, there are two flu seasons each year. The World Health Organization suggests two different vaccines every year—one for each hemisphere.

Many big companies offer flu shots to their workers, sometimes for a small fee or even for free. The yearly vaccine includes parts from three types of flu viruses to help protect against them.

Associated health complications

Having health problems that weaken the body's defenses can make the flu much worse.

People with diabetes need to keep their blood sugars just right. If they don’t, even a small flu can become very serious. That’s why doctors advise them to get a flu shot before flu season starts.

Those who have asthma or lung problems like chronic obstructive pulmonary disease should also get the flu shot early in the season. The flu can cause serious breathing problems for them.

People with cancer often have weaker defenses because of their illness and treatments. They should get the flu shot, not the nasal spray, to stay safe.

Individuals with HIV/AIDS also have weaker immune systems and can get very sick from the flu. The flu shot can help protect them, too.

Cost

The flu season can affect health and the economy. In 2025, the World Health Organization said that seasonal flu can cause deaths around the world.

In the United States, studies have shown that flu can lead to many hospital visits and lost work time. This can cost a lot of money each year. The flu season can also result in many hospital stays and deaths in the country.

Notable occurrences

Seasonal flu can sometimes be very serious, almost as bad as bigger outbreaks called pandemics. Some flu seasons are worse than others.

One notable season was in 1928–1929 in the United States. It was considered very serious at the time.

In 1946–1947, the first flu vaccine didn’t work well. This led to a big outbreak that spread worldwide.

The 1950–1951 season was very bad in England, Wales, and Canada. The United States had a milder season this time.

In 1952–1953, many countries had big flu outbreaks. In the United States, schools closed because so many students and teachers were sick. The outbreak spread to Europe too.

The 1967–1968 season was the last before a new type of flu, called the "Hong Kong flu", started spreading in 1968. It was especially hard in England and France, but milder in North America.

The 2014–2015 season had a lot of flu in the United Kingdom.

The 2017–2018 flu season was very severe in the United States. It was the worst since 2010 and affected almost the whole country.

From March 2020 to Fall 2021, there was no flu season because of measures taken during the COVID-19 pandemic.

The 2024-2025 flu season was one of the worst in many years in the United States.

In June 2025, scientists found a new version of the flu virus. This new version started the flu season earlier than usual in the United Kingdom, Canada, and Japan, and it caused a very severe season.