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One of [NHK's
stories](https://www3.nhk.or.jp/news/easy/k10012410471000/k10012410471000.html)
today is about parents delaying routine vaccines, and it has the following
line:

ワクチンを受けるのが遅く **なると、ならないはずの** 病気になったり、病気がひどくなったりします

I'm interpreting it something like: if you get the vaccine late, then you will
definitely get the illness and the illness might get worse. Or perhaps, you
must not get the vaccine late or you will surely get the illness or it might
get worse.

I feel like I understand ~たりする, and と、ならない but the bold portion above is
really throwing me.
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2020-04-30T19:10:46.620
76962
2020-04-30T22:42:08.150
36714
question
grammar
Conditional or must with はず in と、ならないはずの
Aeon Akechi has largely answered your question with his comment, but I will
expand a little.

In short, the sentence breaks down like this:

> ワクチンを受けるのが遅くなると
>
> If you get the vaccine late
>
> [ ならないはずの病気 ]になったり、病気がひどくなったりします
>
> You will get an illness you (otherwise) wouldn't have, or your illness will
> be worse.

I think you were pretty close to parsing this correctly, but the important
thing is that `ならないはずの` is just qualifying `病気`. This `なる` comes from
[病気になる](https://jisho.org/search/%E7%97%85%E6%B0%97%E3%81%AB%E3%81%AA%E3%82%8B),
so this `ならないはず` is an expression of the expectation that under normal
circumstances this is something you wouldn't catch.

I chose `illness will be worse` over `get worse` because this is talking about
contracting the illness, and presumably not people who already have it.
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2020-04-30T22:42:08.150
76966
2020-04-30T22:42:08.150
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76962
answer