Wow. Thanks for the replies. My thumb is better. Details below. But first…
I am genuinely sorry if I alarmed anyone with my talk of the “flesh eating virus.” It was just a thought that crossed my mind in the early morning while I contemplated a disturbing wound on my thumb. I didn’t mean to cry wolf. The symptoms were exactly as described, but I didn’t actually think I might have a flesh-eating virus (or nasty bacterium. Thanks for the primer, Stranger.)
Regarding suggestions that I look into a first-aid course, and that poking in wounds isn’t a good idea–excellent points, but not applicable in this case. I didn’t think to mention it in my OP, but I am a Wilderness First Responder, which includes training in invasive wound care, including removal and/or management of embedded objects. Technically I’m not supposed to do invasive wound care in a non “wilderness” setting, but hey, it’s my own thumb.
Funny you should mention that. Here’s a play-by-play:
Just before I posted (7 am), I put some Betadine on it and bandaged it. After that point, it didn’t seem to be getting worse, so I went to work and monitored the thumb.
At 2 pm, I realized that it hurt a lot less. I removed the bandage and observed that the swelling and redness had decreased. The area of redness had not spread. So I put some more Betadine on it and another bandage. At 6 pm, the swelling and redness had decreased further. More Betadine and a new bandage.
At 10 pm, I closely examined it. I noticed a tiny black speck. I trimmed some dead skin away from the edges and removed the black speck. Nothing else seemed to be in there. The wound was not deep. Betadine and bandage.
At 9 this morning, I removed the bandage and found that it looked much better. Only minor swelling and pain remained. The redness was almost gone. I trimmed more dead skin and cleaned it thoroughly. Betadine and Bandage.
At this point, 12:30, the redness and swelling is completely gone. Noticeable healing has occurred. It is still somewhat painful when touched, but much less so than before. It is much much much better.
So, I’m guessing that I did in fact get something embedded in my thumb, which then got infected. The Betadine and removal of the item helped curb the infection. The culprit might have been the black speck, or something else that came out unnoticed. I’m planning to keep it bandaged and keep a close eye on it, but so far, I seem to be mostly cured. 
I’ll let you know if anything interesting happens.
On the plus side–this has turned into an interesting and informative thread.
Also, it has gotten me thinking about an aspect of our medical system that could be vastly improved–treatment for all those things that need care in a time frame between “immediately” and “a few days from now.” But that’s a topic for another forum. Maybe I’ll start a thread…