Influenza—commonly known as the flu—is a respiratory virus that can infect people throughout the year. However, flu season, which is when cases increase significantly, typically occurs during the fall and winter months in the United States. It usually begins in October and can last as long as May. Peak months tend to fall between December and February, although this can vary depending on the year and geographic location.

Roughly 8% of the U.S. population gets sick each flu season. Getting an annual flu shot remains one of the most effective ways to reduce your risk of infection.

Why Does Flu Season Peak in the Fall and Winter?

The timing of flu season is influenced largely by colder temperatures, which help the virus spread more efficiently. In the United States, flu activity generally starts increasing in October and can last through May, though the peak may vary by region.

Cold weather contributes to the spread of the virus, especially in areas with longer winters. Additionally, people tend to gather indoors more often during colder months, increasing the likelihood of transmission.

Flu Season Around the World

Globally, flu season occurs at different times depending on your location. In the Southern Hemisphere—such as South America, parts of Africa, Australia, and Antarctica—flu season typically runs from April through September. In the Northern Hemisphere—including North America, Europe, and most of Asia—flu season generally runs from October through May.

Other Illnesses That Peak During Flu Season

Several other respiratory illnesses also spike during the fall and winter. These include:

  • Bronchitis – Inflammation of the bronchial tubes

  • Common cold – Causes runny nose, sneezing, and cough

  • COVID-19 – Involves symptoms like fever, cough, and shortness of breath

  • Pneumonia – Infection that causes fluid or pus in the lungs

  • Respiratory Syncytial Virus (RSV) – Cold-like symptoms, most common in children

Indoor gatherings during the colder months further support the transmission of these viruses.

Who’s at Greater Risk of Getting the Flu?

While anyone can contract the flu, some individuals are at higher risk of severe illness and complications. These include:

  • Young children and older adults

  • People with chronic conditions (e.g., heart disease, lung disease, kidney issues, diabetes, or obesity)

  • Pregnant individuals

  • People with weakened immune systems due to illness or medications

Tips for Preventing the Flu

Although there’s no guaranteed way to prevent the flu, several strategies can reduce your risk. One of the most important is getting the annual flu vaccine. The CDC recommends that everyone aged six months and older receive the flu shot each year.

Other prevention tips include:

  • Avoid close contact with people who are sick

  • Regularly clean and disinfect frequently touched objects and surfaces

  • Cover your mouth and nose with a tissue or your elbow when coughing or sneezing

  • Wash your hands often with soap and water for at least 20 seconds

  • Use hand sanitizer with at least 60% alcohol if soap is unavailable

  • Stay home if you’re feeling sick

When Is the Best Time To Get a Flu Shot?

It’s best to get the flu vaccine before the virus begins circulating widely in your community. September and October are considered ideal months for vaccination. If you get vaccinated too early, your protection may wear off before flu activity peaks. However, it's still beneficial to get the flu shot later in the season, even in January or beyond, as the flu can continue circulating into spring.

A Quick Review

Flu season in the U.S. begins in October, peaks between December and February, and may last until May. While anyone can catch the flu, certain groups are at higher risk of complications, including young children, older adults, pregnant individuals, and those with chronic conditions.

To protect yourself and others, get your flu shot in early fall, practice good hygiene, and stay home if you feel unwell.