Because the influenza virus changes its genetic make-up from year to year, you're most likely to be exposed to a new virus.

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There are three types of flu:

• Type A is a potentially severe illness that easily spreads throughout a population, even globally, affecting a large number of people at the same time.
• Type B is generally less severe and tends to affect fewer people.
• Type C is not considered clinically relevant because it causes very mild symptoms.

Your immune system can fight off a flu virus you've already had or been vaccinated against. However, because the influenza virus changes its genetic make-up from year to year, you're most likely to be exposed to a new virus. That's why you can get the flu every year, and why a flu shot is recommended every year.