Seasonal Influenza (Flu)

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Page last updated 10/30/2026.

The County of San Diego is dedicated to preventing vaccine-preventable diseases by following the West Coast Health Alliance’s (WCHA) recommendations for vaccinations based on credible, transparent, and science-based evidence.

Woman sitting on couch at home sneezing into a tissue.

Influenza (flu) is a contagious respiratory illness caused by influenza viruses.

  • It spreads easily from person to person. 
  • It infects the nose, throat, and sometimes the lungs. 
  • It can cause mild to severe illness. In some cases, it can lead to death. 

On this page: Prevention | Transmission (Spread) | Symptoms | Diagnosis and Treatment | Resources

Transmission (Spread)

Flu spreads mostly through droplets in the air when people with flu cough, sneeze, or talk.

  • These droplets can land in the mouths or noses of people who are nearby. 
  • Less often, a person might get flu by touching a surface or object that has flu virus on it, and then touching their own mouth, nose, or eyes. 

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Symptoms

Flu symptoms usually come on suddenly and may include: 

  • Fever* or chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Body or muscle aches
  • Headache
  • Feeling very tired
  • Vomiting or diarrhea (more common in children)

*It’s important to note that not everyone with flu will have a fever.

Some people may develop complications because of the flu, such as sinus and ear infections to pneumonia. 

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Diagnosis and Treatment

Doctors can test for the flu, but they may also diagnose it based on your symptoms.

  • Most people who get the flu will start to feel better in a few days to less than two weeks.  
  • Most people with flu have mild illness and do not need medical care or take special medications. 
  • If you get sick with flu symptoms, in most cases, you should stay home and avoid contact with other people unless you need medical care.

There are prescription medicines called antiviral drugs that can help treat flu illness.

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Resources

County of San Diego

California Department of Public Health (CDPH)

Centers for Disease Control and Prevention (CDC)

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Call the Immunization Unit at (866) 358-2966, or send an email, for more information.