When I was a boy, I lived in a wonderful neighborhood filled with kids my age. One of these was Skipper Jaffe, who lived two doors down. When I was around 7, Skipper came down with measles. When the news got out in the neighborhood that the Jaffe house had measles, all the other moms did what most moms did in those pre-vaccine days: they sent their kids over there to play. Within days, every kid in the neighborhood had measles. In a week our neighborhood “epidemic” was over.
I doubt these mothers knew the statistics on measles, that the mortality for measles was around 0.1%, about the same as seasonal flu now, and that most of those were in children under 5. I doubt it would have made much difference. They knew that, absent a vaccine, their children were almost certainly going to be exposed to measles and the sooner they were exposed the better to establish future immunity. They did the same for chicken pox.
In 2018, even with an effective vaccine, 140,000 people died of measles worldwide and, again, most of these were children under 5. Measles is a terrible disease in the few who develop a severe infection with sometimes lifelong consequences. There has not been a measles death in the US since 2015 thanks to an effective public health campaign to vaccinate children, but the anti-vaccination movement is producing a population of vulnerable people that will be at risk for measles in the future (unapologetic vaccine plug). https://www.cdc.gov/mmwr/volumes/68/wr/mm6840e2.htm.