Influenza is a viral infection that attacks your respiratory system, including your nose, throat, bronchial tubes and lungs.
If you're generally healthy and you catch influenza — commonly called the flu — you're likely to feel rotten for a few days, but you probably won't develop complications or need hospital care. If you have a weakened immune system or chronic illness, though, influenza can be fatal.
For those at high risk of influenza, the first line of defense is an annual flu shot. This year, in addition to a regular flu shot, you may need a different flu shot for protection against novel H1N1 swine flu, a new type of influenza identified earlier this year.