They are crustaceans , tiny crustaceans. And when I read about them online they are always in the feet. But she thought, being a California girl, that she could lie on a towel in the sand. We don’t have those things here. So she got them in her spine and they are all inside her now, internally. It’s really bad. She has muscles she can’t move, but apparently it isn’t lockjaw.
The sand fleas I see are not “tiny” https://en.wikipedia.org/wiki/Emerita_(genus). I think it’s better referred to as sand fly, which as a bug often is, are indeed very small https://en.wikipedia.org/wiki/Sandfly
Unless it’s possible a nymph “flea” burrowed in, but there is no discussion of them being a medical threat.
Also, watch the interchangeability of these names. Some people apparently call the “Sand fly” a “sand flea”, etc.