My mom had Scarlett Fever as a child - (Central California) - no one else in her very large family or small town had it or got it.
Dr.s think it damaged her thyroid.
She’s 93 now and in great shape.
My sister and I had it back in the 1950’s. We were quarantined; I have vague memories of people dropping a box off at the door and scampering back to their cars.
I got out of it with a heart murmur which hasn’t bothered me over the years.
I’ve asked docs why kids don’t get it these days. Their response was that infections are likely to be treated early with antibiotics, so things don’t progress.