Vaccines are used to boost your immune system and prevent many diseases, some of which are serious or life-threatening. Vaccines “teach“ your body how to defend itself when germs, such as viruses or bacteria, invade it. After exposure to the vaccine, your immune system learns to recognize and attack the viruses or bacteria if you are exposed to them later in life. As a result, you will not become ill. Or, if you do get the illness, you will likely have a milder infection. Vaccines are very safe and very effective at protecting against certain serious diseases.