It means the population as a whole would have herd immunity (on average, every infected person would infect significantly less than 1 new person).
Result=> epidemic dies out.
Result=> epidemic dies out.
The “standard example” for natural herd immunity eradicating a disease is the black plague, but most experts now believe that herd immunity isn’t actually responsible for its disappearance from Europe.
Vaccines get you the individual protection (and eventually herd immunity) without all the death.