Imagine that one day you were captured by aliens and dropped off on a strange new planet. You looked around and saw many strange new creatures. What would you do? You would have to learn which creatures were dangerous and which were friendly. You would have to learn how to protect yourself from the dangerous creatures. That might take some time. While you were learning to fight them off, you would probably get hurt. If your enemy was very strong or if you were very weak, you might even be killed during battle. Defeating a dangerous creature would make you stronger. You would know how to fight it off the next time it came around. In time, you would learn how to stay strong and healthy on that strange new planet. Wouldn't it have been a lot easier if someone had given you a book with instructions on how to identify the dangerous creatures and how to protect yourself from them!
This may sound like a silly story, but this is just what happens in your body when you are born. When you are born, your body enters a strange new land. It is filled with many new sights, sounds, smells, and dangers. Germs are everywhere in our world. Many of them don't bother us. Some of them are even good for us. The germs that we usually hear about, though, are the ones that make us sick. When we are babies, our bodies have never seen germs before. They do not know which ones are friendly and which ones are dangerous. Our bodies learn how to fight bad germs each time they meet a new one. Just as you might get hurt fighting an alien creature, our bodies get hurt when they are learning to fight off germs-we get sick. We are lucky, though. Doctors and scientists have learned how to give our bodies an instruction manual for fighting off the really bad germs. That instruction manual is a vaccine. Vaccines teach your body to recognize and defend itself against certain germs. It is sort of like a training camp inside your body! Your body learns how to fight the enemy, but no one gets hurt. You don't have to get sick!
Most vaccines are given to babies and small children. They prevent dangerous illnesses that used to kill many people. Polio, measles, whooping cough, chicken pox, and many other illnesses are rarely seen now because of vaccines. While most vaccines are given to children, adults sometimes get vaccines, too. The flu shot is a vaccine that many people get every year. You have to get a new shot every year because the flu germ is always changing. It is like a monster with many disguises. Your body might learn how to recognize it one year, but the next year it will have on a different disguise and your body will not recognize it.