Answer:
with some diseases such as small pox, if someone is able to survive against the disease they will be able to have a lifetime immunity from getting that said disease again. with vaccines, they inject a small amount of the disease into you, so that your cells that protect you from that disease will get accustomed to that said disease. with that being said, if there is an infectious disease once they get a vaccine going, if you start getting that vaccination for it at a young age, your body will become "immune" to it, so that when you have kids basically theyll have the genes that already have been introduced to it, so as more and more people stop getting that disease more and more people start becoming "immune" to it, causing it to die out and either become a very rare disease to get or wiped out completely