Rouser2 said:
Sigh.
I just explained it to you.
What Prester John said, and....
If a population has to rely on naturally-acquired immunity to combat infection, disease will tend to come in waves. An epidemic will leave the vast majority of survivors immune, and at that point there will be a low incidence. Maybe the causal organism will even die out for a bit. If you slapped a strict quarantine on the population at this point, you might have a chance of maintaining a disease-free status. But if you're dealing with humans and not farm animals, that's unrealistic.
Gradually the number of non-immune individuals will increase, by new births and by immigration. Eventually the proportion of these susceptible people will become high enough that the organism can spread between them - there are now too many of them to "hide" in the immune majority. Then, without that strict quarantine and assuming that the WHO has not succeeded in eradicating the organism world-wide, the bug will inevitably be re-introduced. And it will cause another epidemic among the non-immune individuals. And back to the beginning for another cycle.
Most people thus get the disease eventually.
The only way to prevent this is to arrange for >90% of the population to become immune
without getting the clinical disease. The only known way to do this is by vaccination. Unless you know of a different way you still haven't been able to explain to us.
With vaccination the population immunity level is kept up at the immediate post-epidemic level, and the organism doesn't get a chance to take hold. But (except for smallpox, to date), it's still lurking out there in places like Haiti, either to re-infect the population if the vaccination percentage falls off, or to pick off unwary unvaccinated individuals who are unwise enough to go there to find it.
Rouser, how else could you achieve a population with >90% of individuals immune, without either vaccinating, or allowing most people to catch the disease?
Rolfe.