Previously, previously, previously, previously.
This all works at the meta level: what proportion of people need to be 'immunized' (by a decent education, I suppose) against the anti-vax bullshit for it not to spread? How does infectiousness of the bullshit depend on how pervasive things like twitter & facebook are, and when was the threshold crossed when there were no longer enough people immunized?
It would help if it wasn't the subject of a propaganda war.
One of the things you'll see in the model is that the infected become permanently resistant if they survive. To what extent (e.g. resistance may expire) that happens will depend on the disease but the basic idea is sound. That's very important to your meta question. When childhood diseases are widespread, most people know what it was like. "Oh yeah, my sister got that and she was sick for like three months, she couldn't even get out of bed". This makes it easier to "sell" immunisation. I was given every immunisation available as a child, because my uncle had been badly affected, so from my mother's point of view obviously you don't want that to happen to your kid, you immunise them, and if you're immunising against one thing, why not everything they've got? Amusingly it didn't work very well, I had several of these diseases anyway in early childhood, and it sucked, immunisation is never 100% effective on individuals.
But anyway, the consequence is that where immunisation is successful you don't have that "scorched earth" effect. If you tell a new parent who has never seen anybody get sick from Measles, let alone die from it, that you aren't sure they should get their kids the jab, you've heard maybe it's bad - they don't have that real world basis to tell you that you're crazy and should fuck off.
You could imagine education would fix this, but er, good luck. Education is not effective enough to hit the rates we need. Most industrialised nations claim to have more or less 100% literacy. They've got mandatory infant education, surely even not-so-bright kids can at least read? But if you go out into their societies and do clandestine measurement well, maybe 5% are functionally illiterate. Now what does "functionally" mean? Well these people can puzzle out a street sign, they can write their name, but they can't read a newspaper article for sense, they can't read the four pages of instructions on the medicine, or the manual for their new car. And that's literacy, which is something we're working on more or less from day one, imagine the reality for something like science education. You will not hit the 90+% "herd immunity" for resistance to nonsense about vaccination.
That sounds right to me. For the first part there must then be a population-dynamics thing – people forget / never experienced the consequences of immunisation rates being low, are less likely to immunise their children, rates fall, people begin to learn the consequences of that, rates rise – which may or may not have a stable equilibrium. For the second part, yes, I guess there's no real hope of education fixing it.
The only actual fix is likely mandatory immunisation which is probably unacceptable, at least in the US I would think.
Mandatory immunization used to be required for school attendance, public or private, before the idiots normalized fraudulent exemptions and MDs played along until it was too late.
See also: Going Critical, by Kevin Simler:
Not sure why the embed to this work in progress is apparently broken.