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    Flu

    Influenza, or the flu, is a contagious illness caused by flu viruses.

    Influenza (flu) is a respiratory virus

    Influenza, or the flu, is a contagious illness caused by flu viruses. It affects the nose, throat, and lungs. The flu is caused by different types of viruses called influenza A and influenza B. In the United States, the number of people with flu usually starts increasing in the fall, with the highest number in the winter.

    People most at risk of severe illness

    Young children, older adults, pregnant people, and people with chronic health conditions (like asthma, diabetes, or heart disease) are most at risk of severe illness. Influenza causes many people to be hospitalized and to die every year in the U.S. CDC estimates that from October 1, 2024, until May 17, 2025, there were at least 610,000 hospitalizations and 27,000 deaths due to flu. This is the lower end of a range, and likely an underestimate.

    Symptoms of flu

    • Fever with or without chills
    • Headache 
    • Tiredness
    • Cough
    • Runny or stuffy nose
    • Sore throat 
    • Body aches
    • Sometimes vomiting and/or diarrhea (more common in children)

    Flu spreads through tiny droplets before you even know you are sick

    The flu spreads through tiny droplets when people with the flu cough, sneeze, or talk. You can also get it by touching surfaces with the flu virus on them and then touching your face. Cover your mouth when you cough or sneeze and avoid touching your face.

    Incubation and infectious period

    The incubation period is the time from when you are exposed to the flu virus to when you start showing symptoms. You might get sick one to five days after being exposed, however,  for most people symptoms start within 48 hours of exposure.

    The infectious period is the time when you can spread the flu virus to others. People who have the flu but have no symptoms can still spread it. Those with flu can spread it one day before and for five to seven days after feeling sick. People with flu are most contagious in the first three to four days after symptoms begin. They remain contagious until the fever is gone (without fever-reducing medicine) and they begin to feel better.

    Incubation and infectious period

    Influenza prevention

    Learn how flu spreads and take these prevention steps:

    Testing

    Health care providers can swab your nose or throat and test for flu. It is important to get tested so that you can get the proper treatment.

    Treatment

    • Rest: Get plenty of sleep.
    • Fluids: Drink lots of water, juice, or clear broth.
    • Medications: Over-the-counter medicines can help relieve fever and other symptoms. A doctor can prescribe antivirals.

    Antivirals

    Sometimes, doctors prescribe antiviral drugs to treat the flu. Antibiotics don’t work because flu is caused by a virus, not bacteria. Antivirals can help you get better a little faster and keep you from getting really sick.