A bunch of them are fundamentalist wackos that refuse vaccinations. I'm happy their numbers are being reduced naturally. But women are broodmares in almost all christian splinter cells, so they tend to repopulate quickly.

Vaccinations against these diseases can also decrease in affectiveness with age. Its not often done, but a titer can be done to see if you are still immune. I got one when switching to a new doctor and found I was no longer immune to measles and had to get the vaccination.