The professor doesn’t talk because of the disease. I’m sure now. He probably does it because it slows the disease, perhaps... it might explain why he lasted longer than others despite not having special disease immunity or anything.
I swear, every chapter is the perfect blend of the quote by Marcus Aurelius “Μὴ καταφρόνει θανάτου, ἀλλὰ εὐαρέστει αὐτῷ, ὡς καὶ τούτου ἑνὸς ὄντος ὧν ἡ φύσις ἐθέλει.” (Think favorably of death.)