If you've been vaccinated against measles, you won't get measles. how does that work?

I love questions like this! Science is so cool! So, strangely enough, a vaccine is a very weak bacteria. If the bacteria was strong, your immune system couldn't defend. Because its a weak cell, your immune cells can fight it. Did you know that you have a special cell inside you? This cell remembers every bacteria that comes to your body. Because you had very weak, damaged bacteria in you, (measles bacteria), you can remember it, and grow strong against it. Then, when germs try to enter your body, your immune system is right there to defend it. Everytime the bacteria of Measles tries to enter, your immune system is extra strong because it remembers it. Isn't that cool? 
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From one Smartie to Another -  BubbleSmartie11