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Symptoms of the Flu

Influenza (or the flu) is a contagious respiratory (breathing) virus that can cause many mild to severe symptoms and breathing problems.

When you’re feeling sick, it’s important to know which illness you have so you can get the best care. Banner Health can help guide you on everything from flu symptoms to watch out for, to how the flu differs from other viruses like the common cold, COVID-19 and even “stomach flu” (viral gastroenteritis).

What are the symptoms of the flu?

Influenza viruses cause the flu. These viruses may cause a range of symptoms that often appear suddenly and quickly get worse. For example, you may feel fine in the morning but feel very sick by the afternoon.

Common flu symptoms include the following:

  • Fever (usually above 100.4°F or 38°C  )
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Fatigue (extreme tiredness)
  • Headache
  • Chills
  • Nausea, vomiting or diarrhea (more common in children)

How long do flu symptoms last?

Flu symptoms often appear within one to four days after exposure to a flu virus. These symptoms may last for several days and up to two weeks.

Can the flu cause serious health complications?

Most people who get the flu will get better within one to two weeks. Still, some people are at an increased risk of severe complications, including young children, pregnant people, older adults and people with chronic medical conditions and/or weakened immune systems.

These complications can include:

What is the difference between the flu and other illnesses?

At first, the flu can often be mistaken for a bad cold or out-of-control allergies. Here is how the flu differs from other health conditions and illnesses.

Common cold

The flu and the common cold share similar symptoms, such as a runny or stuffy nose, cough and sore throat. However, flu symptoms are more severe and appear more suddenly than a common cold. Fever, body aches and fatigue are also stronger with the flu.


COVID-19 and the flu are both respiratory illnesses caused by different viruses. While they share some symptoms like fever, cough and fatigue, COVID-19 causes more symptoms than the flu, including loss of taste or smell, shortness of breath and digestive issues.

COVID-19 has a higher risk of severe illness and complications than the flu.

The best way to determine which virus you have is to take a rapid or PCR test.


Triggered by exposure to allergens like pollen, dust or pet dander, allergies can cause symptoms that resemble the flu, such as a runny or stuffy nose, sneezing and coughing. However, allergy symptoms typically include itchy or watery eyes and don’t cause fever or body aches.


The flu is a viral infection, while pneumonia is a lung infection that can develop from bacterial or viral infection. The flu can sometimes progress to pneumonia, which can cause symptoms like the flu but with additional and often more severe breathing problems.

The flu tends to come on suddenly, while pneumonia develops more slowly. Muscle aches and fatigue accompany the flu, while pneumonia symptoms tend to center around the lungs.

Stomach flu (viral gastroenteritis)

Stomach flu (also known as viral gastroenteritis) is caused by viruses that primarily affect the digestive (gastrointestinal) system. It leads to symptoms like nausea, vomiting, diarrhea and stomach cramps. Unlike the flu, stomach flu rarely causes chest problems like cough or sore throat.

When to see your health care provider

While most cases of the flu can be managed at home, there are instances when medical attention is necessary.

Contact your health care provider or visit your nearest urgent care if you or a loved one experiences the following:

  • A fever that gets better then suddenly worse
  • Signs of dehydration
  • Symptoms don’t improve within two weeks
  • Hoarse, barking cough
  • Combination of fever with a sore throat, headache, stomachache, earache or muscle ache that doesn’t get better

If you have flu-like symptoms and are at high risk for complications, you should contact your health care provider for care as soon as possible.

Learn more about when to seek medical treatment.

It’s not too late to get vaccinated

Even if you know you’ve recently had the flu, there are still many good reasons to get a flu shot. That’s because several strains of the flu circulate each season.

Catching the flu does give you some protection, but the flu shot boosts your defense against the most widespread strains of the year. This can help keep you healthy for the rest of the season.

Learn more about flu vaccination and common myths and facts about flu shots.

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