so you know this as "fact"? What do you base your decisions on?
There are and have been massive epidemics, in nations that haven't had their populations vaccinated and those people spread these diseases, when they travel, as well as unvaccinated travelers to those outposts.
Germs, bacteria, and viruses just don't disappear......they hang around waiting to attack those who aren't protected. Immunity is built up by having many diseases, though a very few people do contract such a disease a second time, even though they've already had it before; usually less virulently than the first time. For instance, I did have regular measles twice, in the space of five years. The first time damned near killed me; the second time around it was no bid deal at all. And no, the second bout was NOT German Measles/Rubella, but the real deal.