Render of Flu (Influenza)

What Is Flu?

Influenza (in-floo-en-zuh), or flu, is a contagious viral infection that can cause mild to severe symptoms and life-threatening complications, including death, even in healthy children and adults.

Influenza viruses spread mainly from one individual to another through coughing or sneezing. Less often, they can also spread through touching a contaminated surface and then touching the mouth, eyes, or nose. Individuals can pass flu on to others even before their own symptoms start and for a week or more after symptoms begin.

June 2024 Update: Avian influenza (H5N1 bird flu) is widespread in wild birds worldwide and is causing outbreaks in poultry and US dairy cows. While the current risk to the public is low, CDC is working with states to monitor for activity and provide ongoing updates.

Bird Flu. Noun. A disease caused by influenza (flu) viruses that can infect birds and other animals, including cows, dogs, and cats. While rare, bird flu can infect people who are in close contact with sick birds or animals. Also known as avian influenza.


While the numbers vary, in the US, millions of individuals get sick, hundreds of thousands are hospitalized, and tens of thousands die from flu and related complications each year. The 2023-2024 season resulted in an estimated 35 million illnesses, 390,000 hospitalizations, and 24,000 deaths in the US, including 173 pediatric deaths.

Flu also affects employers and businesses and costs an estimated $11.2 billion in direct and indirect costs in the US annually. During the 2022-2023 respiratory season, CDC estimates that flu vaccination prevented 6 million flu-related illnesses in the US, 2.9 million medical visits, 65,000 hospitalizations, and 3,700 deaths.


Flu is not just a common cold. It usually comes on suddenly, and people with flu may have some or all of the following symptoms (think F.A.C.T.S.):

  • Fever
  • Aches (muscle, body, and headaches)
  • Chills
  • Tiredness (fatigue)
  • Sudden onset
  • Cough, runny or stuffy nose, and/or sore throat
  • Vomiting and diarrhea (more common in children than adults)


The best way to prevent flu is to receive an influenza vaccination every year. The Centers for Disease Control and Prevention (CDC) recommends that everyone age 6 months and older get vaccinated annually. The best time to get vaccinated is in the fall, before influenza viruses begin spreading in your community. However, vaccination throughout the flu season is still beneficial.

Flu vaccines are updated annually to protect against the influenza viruses that are most likely to circulate during the upcoming season. Flu vaccines can vary in how well they work, but even in cases when flu vaccination does not prevent infection completely, it can reduce the severity and duration of disease and prevent serious complications. For more than 50 years, hundreds of millions of individuals in the US have safely received seasonal flu vaccines.

View CDC flu vaccination recommendations for the 2023-2024 flu season


Annual flu vaccination is the best way to prevent flu. Antiviral drugs are not a substitute for annual flu vaccination; however, prescription antiviral medications serve as an additional line of defense. CDC recommends that all individuals who are hospitalized, severely ill, or at high risk for developing serious flu-related complications should be treated with antiviral drugs immediately if flu is suspected.

Treatment of flu with antiviral drugs can reduce influenza symptoms, shorten the duration of illness by one to two days, and prevent serious complications, like pneumonia. Antivirals work best when taken within 48 hours of getting sick, but may still be beneficial when given later in the course of illness. Healthcare professionals may treat patients based on their clinical judgment and knowledge about the level of local flu activity.

Learn more about treatment options


Reviewed May 2024

Source: Centers for Disease Control and Prevention

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Bird Flu. Noun. A disease caused by influenza (flu) viruses that can infect birds and other animals, including cows, dogs, and cats. While rare, bird flu can infect people who are in close contact with sick birds or animals. Also known as avian influenza.

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