The flu is a highly contagious illness caused by an influenza virus. The influenza virus causes infections of the nose, throat, and lungs. It spreads easily by coughing, sneezing, and direct contact with spit, phlegm or runny nose.


Flu symptoms appear 1 to 4 days after exposure to the virus. They generally include the sudden appearance of some or all the following:

  • fever
  • cough
  • muscle aches and pain
  • headache
  • chills
  • fatigue (tiredness)
  • loss of appetite
  • sore throat
  • runny or stuffy nose
  • diarrhea, nausea or vomiting (more common in children)


The influenza vaccine is called the flu shot. It is important to get a flu shot every ‘flu season’, because the virus changes and each vaccine is designed to address the changes.

Vaccinations protect you as well as those around you by helping your immune system recognize and fight the flu. There are different ways a vaccine can do this. Some vaccines introduce a weakened virus or an inactivated virus into the cells, while other vaccines create a copy of the virus or introduce an empty ‘shell’ without a virus inside it.

No matter how the vaccine works, the vaccine itself will not cause the flu. The flu shot helps the body use its own immunity to prevent the flu.

People who should get the flu shot: Everyone older than 6 months should receive the influenza vaccine when available.

People who should not get a flu shot are those who have

  • experienced anaphylactic reactions to previous influenza vaccines or
  • developed Guillain-Barré Syndrome (GBS) within 6 weeks of a previous influenza vaccination.