Not everybody gains natural immunity to CP after having had it previously. MOST do, but there are always the exceptions that prove the rule!
My DS1 had CP so badly at 11mo that you could do dot-to-dot on him and my then 4yo DD and her friend did while I was on the loo!, he even had it inside his eyelids.
He got it again when he was 4y11mo. Almost 4 years to the day later, when my DS2 got them!
He had them just as badly the second time too.
I wasn't surprised though - I personally have had CP 4 times now. Once at 5yo, again at 8yo, then at 19, then at 27.
The jab wouldn't help either - because it works on the premise that you get natural immunity to CP from the jab, and that won't work if CP itself didn't cause enough of an immune system response to make you immune!
I am living in dread of DS3 catching CP, because a) I will no doubt get it again, or even worse, shingles. B) DS1 will no doubt get it again. C) Due to having a compromised immune system due to steroid meds for asthma, CP is VERY dangerous for DS2 now.
I'm looking at paying for DS3 to be immunised against CP!