I never thought about it this way:

1. Infectious diseases (smallpox, polio etc) inflict horror on the population.
2. Vaccines are invented and make the population safe and healthy.
3. The population never sees the horrors of what infectious diseases can do
   4.Populations start to think the vaccines are not needed.
4. Horrible disease and death comes back with a vengeance.

There seems to be small sporadic outbreaks of infectious diseases lately so I hope that step 5 never really takes hold.