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What are influenza viruses?

Influenza viruses can cause many infections, including flu, swine flu and human parainfluenza viruses.


Influenza, or flu, is a respiratory infection caused by a number of influenza viruses. Flu affects the nose, throat and lungs and can range from mild to severe. It can become serious or even fatal in young children, the elderly and people with certain health conditions.

Swine flu

Swine flu, also known as H1N1, is a variant flu virus that affects humans. It is a mutation of a flu virus that affected only pigs. Its symptoms are very similar to those of the seasonal flu.

Human parainfluenza viruses

Four viruses make up the group of human parainfluenza viruses (HPIVs). They are not the same as flu viruses. The annual flu vaccine will not prevent HPIVs. They can cause respiratory infections, including:

  • Bronchiolitis
  • Bronchitis
  • Croup
  • Pneumonia


Seasonal flu and swine flu have similar symptoms, including:

  • Fever and chills
  • Cough
  • Sore throat
  • Body aches
  • Headache
  • Fatigue
  • Diarrhea and vomiting (possible)

In addition to these symptoms, human parainfluenza viruses also show the following:

  • Chest pain
  • Croup
  • Runny or stuffy nose
  • Shortness of breath
  • Ear infection

How is it diagnosed?

Often, physicians can diagnose flu just by evaluating your symptoms in a physical exam. People who have a higher risk of complications from flu may need a nose or throat swab to identify the type of flu virus. If you have more severe symptoms, your physician may decide that you need further tests to determine the right treatment plan. Some tests that your physician may recommend include:

  • X-ray or CT scan of your chest to look for problems in your lungs such as pneumonia
  • X-ray of your neck to look for swelling in the throat that may indicate croup


All types of flu are very contagious. They spread from person to person through contact with the virus on surfaces or in the air in mucus and saliva droplets from sneezing and coughing. You can avoid getting a flu virus by:

  • Getting the annual flu vaccine to prevent seasonal flu and swine flu (but not human parainfluenza viruses)
  • Washing your hands thoroughly and frequently
  • Avoiding touching your eyes, nose or mouth as much as possible
  • Avoiding close contact with sick people


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