Do you guys even know what a vaccine is? For the Flu, they give you a weak strain of the Flu so your body will be able to kill it quickly and then it will know how to kill it if you ever get it again. Otherwise, when you do get the Flu, your body won't know how to deal with it and that poses a big threat to your health.
I knew that, thanks to Bio, but still, my Mom makes sure I take no vaccines, or anything, I have the best immune system, I am never sick, and havent missed school since 7th grade, I guess I am uber lucky ;D