I suspect that I got it from produce I was chopping for the stock Thursday or soup Friday. I had washed off the celery for the soup, but instead of pulling off each stalk, I cut off the bottom end--then decided those stubs at the bottom would be a tasty snack. It's quite possible that the initial wash had not penetrated to the interior of that end. I remember thinking, as I started to eat them that maybe I should give them a scrub--but they looked clean.
All is calm now, and I've gradually introduced regular foods without any sign of trouble. I'm not ready for a hot fudge sundae yet, but I'm thinking the homemade vegetable soup would be a good supper. (Two people who didn't get sick ate multiple servings of the soup, so it's not suspect. I'm the only one who ate raw celery.)