Newborn infants can get polio without any serious effects, generally. I think the age limit is something like 6 weeks or so, IIRC. If you get it while younger than that, I’ve read it just looks like a bad cold. AND, you will be immune to polio thereafter!
The polio virus is said to be extremely common (or at least was before everybody was getting vaccinated). I don’t know if it still persists in the environments, without there being people to carry it.
In the early part of the 20th century or so, before people had much notion of general hygiene, babies were allowed to crawl around on the floor or whatever, and it was near universal that they would get polio very quickly, before the age limit, and be over it. It’s a common theory (although I’ve also read that some people think this is bullshit), that once parents became careful to keep their babies in cribs or playpens, off the floor, then babies stopped getting polio. So they stopped getting naturally immunized while it was still relatively harmless. Thereafter, some children, or even adults, got polio at later ages, with the horrible results that everyone knows about. That is when and why polio became a horrific epidemic in the mid-20th century, until the vaccine was developed.
I was a fairly young child (about nursery-school age, IIRC) when the Salk vaccine became available. There were massive public campaigns to get everybody vaccinated. Massive public inoculation clinics were held, often in school or church auditoriums. Families lined up for blocks (just like we did in the mid-1970’s to see the first Star Wars) to get their shots. I remember those.
The shots supposedly didn’t last forever though, and it was said that you would need periodic booster shots to keep the immunity up. Therefore, it was a big thing just a few years later when the Sabin oral vaccine came out. For whatever reason, the Sabin vaccine was supposedly good for life, with no need for boosters. So, when the Sabin vaccine came out, there was another massive public campaign to get everybody to get it, even if we already had our shots. So I remember lining up for that one also. The vaccine was served on a sugar cube.