Flu season

Flu season is a time each year when many people get sick with the flu, a sickness caused by a virus called influenza. This usually happens during the colder months in each part of the world, 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.

Every year, the flu makes about 3 to 5 million people very sick and causes between 290,000 to 650,000 deaths worldwide. That’s why 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. Cold temperatures might also help the virus stay alive longer on surfaces. Vitamin D, which our bodies make with sunlight, can also affect our immune system and might change with the seasons.

Timing

Seasonal variation in deaths due to influenza or pneumonia in 122 U.S. cities, as a proportion of all causes

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 reduce the impact of flu season 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.

Pediatric deaths from influenza 2011–2014 - a small fraction of the overall number of flu deaths in the U.S. each year, most of which occur in elderly people[needs update]

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 very 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

During the 2011–2014 seasons, flu-like illnesses have typically accounted for 6% of outpatient visits at the peak of the season in the United States.

The flu season can have serious effects on health and the economy. In 2025, the World Health Organization estimated that seasonal flu causes many deaths around the world each year.

In the United States, studies have shown that flu can lead to many hospital visits and lost work time, costing billions of dollars each year. The flu season often results in a large number of hospital stays and deaths in the country as well.

Notable occurrences

Seasonal flu can sometimes be very serious, almost as bad as bigger outbreaks called pandemics. What makes a flu season worse than another isn’t always clear, but some seasons stand out because they were especially harsh or happened in unusual ways.

One notable season was in 1928–1929 in the United States, which had around 50,000 extra deaths from flu and pneumonia. This was considered very serious at the time.

In 1946–1947, the first flu vaccine didn’t work well. It had protected people before, but during this season, it failed. This led to a big outbreak that spread worldwide, though it wasn’t very deadly. Scientists later learned that the flu virus had changed in a way that made the vaccine ineffective, teaching experts that vaccines need updates when the virus changes.

The 1950–1951 season was very bad in England, Wales, and Canada, with more deaths than in later big outbreaks. 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. The main type of flu that year was especially strong and hard to prevent with the vaccine.

From March 2020 to Fall 2021, there was no flu season because of measures taken during the COVID-19 pandemic. This caused one type of flu virus to disappear completely.

The 2024-2025 flu season was one of the worst in many years in the United States, with many hospitalizations and more infections than any season since the early 2020s.

In June 2025, scientists found a new version of the flu virus with seven changes. This new version started the flu season earlier than usual in the United Kingdom, Canada, and Japan, and it caused a very severe season. This raised concerns about having two very bad flu seasons one after the other, which is rare. This new version of the virus was able to spread a bit more easily than typical flu viruses.