The flu is an unwelcome visitor and brings unpleasant "gifts:" fever, achy muscles, runny nose, dry cough, and sore throat. There is a vaccine that comes out every year in the Autumn, but how does it work? And why does it come out only once a year?
What we call "the flu bug" is really many different viruses. There are two main families: A and B, and both have many individual members, like Influenza A-Moscow, Influenza A-Panama and Influenza B-Sichuan. If you get sick from A-Moscow, you'll develop immunity to it and to its close relatives, but that immunity won't help you fight off A-Panama or B-Sichuan.
With so many viruses, how can a single shot prevent the flu? Well, the flu shot is re-formulated every summer to include what researchers predict will be that winter's three most popular viruses. The shot can give immunity to these three and to their close relatives. However, you could still get sick from a different strain of flu, proving that there are troublesome members in every family.