Epidemics are noisy... This graph doesn't reflect that. Consider this. A ton of people get polio. Then we have herd immunity for 25 years until the percentage o

Epidemics are noisy... This graph doesn't reflect that. Consider this. A ton of people get polio. Then we have herd immunity for 25 years until the percentage of the population that hasn't been exposed rises. Then we have a huge epidemic again. Since the vaccine, we haven't seen a large epidemic except in populations without high vaccination rates.