That being said, I was very confused most of yesterday. I definitely had symptoms that were flu-like, including fever, which pointed at the influenza virus. But the antibiotic was working, which indicated some kind of bacterial infection. When I woke up this morning, though, I realized that the answer was obvious: I had both! The flu must have weakened my immune system enough that the sinus infection I'd had three weeks ago could re-establish itself. This then likely weakened my immune system even more, which made both infections worse than they would have been separately—thus, from Monday to Thursday morning, I was getting worse rather than better. When the antibiotic started to work on the sinus infection, however, that gave my immune system some breathing room to start fighting the virus off.*
I still feel like hell. But I no longer feel like I'm dying.
* All of this is pure conjecture. Although it's based on a fairly sound understanding of basic human pathology, I'd have to have multiple tests and cultures done to verify any of it. The train of reasoning I used to arrive at the referenced conclusion should be considered a result of my somewhat obsessive need to feel like I understand things rather than any kind of true diagnosis.**
** I'm actually a bit proud of myself for being able to admit this. There was a time when I would have defended such an armchair, back-of-the-envelope diagnosis as if it were fact. I'm very glad that I've been able to make progress in my ability to recognize when I might be bullshitting myself. I guess the next step is getting to the point where I don't feel the need to bullshit myself...