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Signs and Symptoms

For most people, the seasonal flu is a moderate illness that causes symptoms such as:

  • Fatigue
  • Fever, chills
  • Stuffy nose
  • Sore throat
  • Headache
  • Muscles aches
  • Weakness
  • Cough
  • With some strains, diarrhea and vomiting

These symptoms may also be seen with a variety of other conditions and seasonal viral infections.

Influenza, however, can be severe and lead to complications such as viral pneumonia or secondary bacterial pneumonia (see Pneumonia) in the very young, the elderly, in those who are pregnant, and in those with underlying conditions such as asthma, lung disease, heart disease, diabetes, kidney disease, liver disease, and in those with weakened (compromised) immune systems as with cancer or HIV/AIDS.

The average time from first exposure to illness (incubation period) for influenza is about two days, and it ranges from one to four days. Uncomplicated symptoms usually last about three to seven days. People shed virus and are infectious about a day before symptoms emerge and then for about five to ten days, or until about 24 hours after their fever ends. Children and those with compromised immune systems may be infectious for longer.

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