What is influenza (flu)?

The flu is a respiratory infection caused by a virus (germ). Influenza occurs most often during the winter and easily spreads from person to person. The "flu season" in the Northern hemisphere runs from October to May and usually peaks between December and February.

Most people who get influenza feel sick for a week or two and recover. In some people, the flu leads to more serious lung infections or to worsening of underlying conditions, such as heart failure or emphysema.

What are the symptoms of influenza (flu)?

  • Sudden onset of moderate to high fever.
  • Dry cough.
  • Headache.
  • Sore throat.
  • Chills.
  • Runny nose.
  • Loss of appetite.
  • Muscle aches.
  • Tiredness.

Many conditions — such as a common cold, diarrhea, and vomiting — are called "the flu," but are not really influenza. “Stomach flu” is a misnomer, since viruses other than the flu cause such illness.

How can you tell the difference between a common cold and influenza (flu)?

Many cold and flu symptoms are similar. Both the common cold and the flu are caused by viruses.

There are some differences with influenza. Symptoms of influenza often hit suddenly and cause you to become weaker and weaker. While the more uncomfortable symptoms of flu generally last from three to seven days, the dry cough and fatigue of influenza can last two to three weeks. Signs that influenza is getting worse include increasing degree of fever, and shortness of breath. If you think that your illness is getting worse, contact your doctor right away.

Symptoms of colds and flu

SymptomColdFlu
FeverAdults-rare; children-sometimesHigh fever (100°F up to 104°F in adults and 106°F in children); can last 3 to 4 days)

Runny nose

Common (Nasal discharge may have a yellow- or green-colored tint. This does not mean you have a bacterial infection.)Sometimes
Stuffy noseCommon

Sometimes

HeadacheSometimes (usually mild)Common (usually severe)
Body aches and painsSometimes (usually mild)Common (can be severe)
Fatigue (tiredness), weaknessSometimes (usually mild)Common (can last up to 2-3 weeks)
Chills, sweatingNot commonCommon
NauseaUncommonCommon in children
Loss of appetiteSometimesCommon
SneezingCommonSometimes
CoughCommon (mild to moderate)Common (can be intense, severe)
Sore throatCommonSometimes
Chest congestion, discomfortCommon (mild to moderate)Common (can be severe)
VomitingNot commonSometimes (more in children)
DiarrheaNot commonSometimes (more in children)
Watery eyesCommonSometimes
ComplicationsSinus congestion, earacheBronchitis, pneumonia (can be life-threatening)

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