I remember it well: our dark winter of itch when the kids were small. It started with a note sent home—years before the pandemic—that my child had been exposed, not to a deadly virus but to lice.

The twinge begins in the afternoon: toes. At my desk, toes, itching. Toes, toes, toes. I don’t normally think about my toes. But as I commute home in a crowded subway car, my feet are burning, and I ...