Sort of a good news / bad news situation from the vet, though in this particular case the good and bad news are the same thing:
We had more or less ruled out fleas previously, on the grounds that I've lived with fleas before, long ago, in Texas, and I remembered that vividly enough that I figured it would be perfectly obvious. But we hadn't seen any fleas jumping around or on us. We hadn't seen any fleas on Sheba.1 I've been itchy a lot, but the flea bites I remember from Texas were big angry red things that lasted for days, and I haven't seen anything remotely like that, so I assumed that whatever the problem was, it couldn't have been fleas.2 We still don't know what's going on with me, but it kind of goes without saying that fleas seem much more plausible now.
This is bad insofar as it's not the most desirable problem to have, and now there's a lot of work to be done as far as vacuuming and washing, but on the other hand, it was easily diagnosed and should be relatively easy to solve. Sheba doesn't have some exotic form of dog leukemia, she's not allergic to mold in the walls that would cost us thousands of dollars to fix, and she's not been driven to emo self-harm because we're not walking her enough.3 Just fleas. Could have been worse. And should it happen again, we'll recognize it much earlier, and won't have to let it reach this point a second time.
Anyway. Since I sort of promised plants with the title, and since I've just made you think about fleas for a few minutes, here are some pictures of Papaver orientale:
2 In retrospect, it seems sort of obvious that Texas would have different kinds of fleas than Iowa, given the differences in climate and possible reservoir species. A flea that lives off lizards and armadillos in a region that rarely freezes would probably have different properties than a deer/squirrel/rabbit flea that has to survive winters of -10F / -23C.
3 (Though we are still going to try to walk her more.)