Lice
granta.com
granta.com
The result is that not everyone goes through live, only few unlucky kids do. Basically, it prevents spread. The lice themselves keep existing, cause spread is not zero.
[1] https://www.newscientist.com/article/dn16997-freedom-from-li...
But it certainly is not as big a problem as it once was, when I was a kid 30+ years ago you still got TV ads about anti-lice shampoo, as it was still a common enough problem. This is definitely not the case anymore.
Except, surprisingly, Sweden where I live now. Lice outbreaks at schools are a very common thing. This surprised me, as this wasn't a thing even in my own ass-backwards country of Moldova.
It ends up being the poorest of the poor who keep alive vermin and other health issues. If they admit to having such issues, we punish them rather than help them resolve it.
Education is a known means to combat poverty, probably because it makes feasible to find what I think of as "middle class solutions" to myriad issues, including health and hygiene issues. Poor people often have a lack of knowledge and that lack of knowledge fosters problems and there are systemic barriers to educating them because we punish them rather than help them if they admit to having certain kinds of problems.
So they hide them and they don't get educated and so forth.
You sometimes see ugly jokes about "The third world country of America" because there are parts of the US where poverty is rampant, infrastructure is falling apart, etc. I imagine most first world countries have some areas of intractable poverty (though perhaps not all -- some stories suggest some countries are successfully resolving issues like homelessness).