Yeah, mold was a VERY large concern when I first found it. That's the main reason I tore into the wall so quickly to look at it. Fortunately, however, I was unable to find any indication of growth. There was some discoloration around the hole itself, but everything inside the wall looked clean.
Actually, thinking about it, it's a tiny leak. Most of mom's bedroom sits on another slab on top of the foundation, including the shower. If there had been a significant leak, it would have eaten that wall up right away, as the slab slopes ever so slightly toward that wall. It also hasn't been there too long. I remember that wall looking just fine when I went off to school after spring break, and I just noticed the damage two weeks ago. I don't know how long it takes drywall to degrade, but I suspect that it hasn't been leaking very long. Of course, I stopped all showering activity in that bathroom immediately after I noticed it, and it was bone dry when I got home last week.
I will, however, keep your advice in mind. I have fixed up the leak problem, so anything growing in there shouldn't get any bigger, and there's no reason why I can't do a little scrubbing followed by a bit of bleach and water treatment.
Edited to add:
On further inspection, this time with a good flashlight, I still see no mold, and the hole is good and dry.
I'm wondering if I can just patch it. The wall isn't actually hollow right behind the hole. It is right up against the slab of the raised floor in mom's room. It isn't more than an inch deep. For a moment, I considered going to the garage for the bondo. I have seen it used in household applications, but I'm wondering if that mud stuff might be better. I don't mind a little shrinkage when it hardens, as I can always go back in for another coat, and I suppose I can build it in layers with adhesive fiberglass tape. maybe strip it down to the wallboard for an inch or so around the hole, dam it with tape, and fill behind it. Do that in layers until it's sealed, and then slap on a texture patch with some paint? It sounds like a decent idea, and it's a whole lot easier than cutting out drywall. I want to fix it permanently, and I want to do it right. Does filling it in sound like an acceptable way to fix it?