I believe the general level of hygiene has declined in the last 50-60 years. When I was in grammar school I never heard of anyone having lice. Now I regularly see treatments for sale, advertised etc.
It might have something to do with the Hippie movement, maybe so many illegals. Maybe the fact that so many diseases can be cured that people don’t have to be as careful.
My guess would be the 50’s was probably the cleanest period in our history.
I don't recall anyone having lice when I was in school (1940s). However, my mother said it was quite common when she was in school (1920s). If a problem seems to go away,people think they no longer have to take precautions against it. So it comes back.