Ehm, regarding polio?
Herd immunity very much was a thing. Nearly everybody got infected by polio when still a baby, at a time where the virus couldn't do a lot of damage to the child. It's only because cleanliness standards at home increased that most children no longer got infected at a time the disease was mostly harmless to them that it got so dangerous to the population.
Of course, that just basically meant that before those standards changed everybody acquired immunity to polio in a manner not unlike a thorough vaccination campaign...
Herd immunity very much was a thing. Nearly everybody got infected by polio when still a baby, at a time where the virus couldn't do a lot of damage to the child. It's only because cleanliness standards at home increased that most children no longer got infected at a time the disease was mostly harmless to them that it got so dangerous to the population.
Of course, that just basically meant that before those standards changed everybody acquired immunity to polio in a manner not unlike a thorough vaccination campaign...