Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School



Flu can cause a variety of symptoms. They can be mild or severe. Symptoms and severity depend on the type of virus, your age and overall health.

Although it is a respiratory virus, flu can affect other body systems. This makes you feel sick all over. Symptoms can include any or all of the following:

  • Chills

  • Moderate to high fever (101 to 103 degrees Fahrenheit)

  • Sore throat

  • Runny nose

  • Muscle aches

  • Headaches

  • Fatigue

  • Cough

  • Diarrhea

  • Dizziness

Dangerous complications also can develop from flu. One of the most feared complications is a bacterial superinfection. A superinfection occurs when the influenza virus attacks a lung and weakens its defenses. This makes the body susceptible to bacterial pneumonia.

Certain people are especially vulnerable to complications. These include:

  • People older than 50

  • Infants

  • People with certain chronic diseases

  • People with suppressed immune systems

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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