I saw Dr. B today for the last time: He says I'm fine, and don't need to take antibiotics anymore. In fact, the ones I've been taking for the last three months were only precautionary -- just in case my brain abscess was caused by tuberculosis, which is still a possibility. He thinks that it was probably nocardia, though he wouldn't rule out TB completely -- in short, it's still a mystery, partly because there was no evidence that my immune system was compromised. But he doesn't need to see me again unless something new flares up.
I can live with being a mystery.