Yesterday, the missus was talking to a set of highly educated and sophisticated young folks — all around or under 40 — and discovered (don't ask me how) that they all knew the word louse only in the figurative sense, as applied to human beings: "a contemptible person", as Merriam-Webster's has it.
They knew the plural form lice for the little critters pictured here, but not singular louse. At least one of them (probably the youngest) went to a school where she knew a kid who had head lice, so the creature isn't foreign to them either. It seems clear that the main meaning is now the human one, but Mrs. V was a little surprised that they'd lost the old, basic meaning entirely.
It's surely the case that most Americans are utterly unaware that lousy originally meant 'louse infested', and now louse is going the same way. (M-W does have that meaning of the adjective listed first still, by the way.)
Well, hey, I did say a tiny example of language change.
Image from here.