Here is one passage I want to be sure we talk about:
QuoteBut the most wretched and deplorable thing of all, at this time, was that when once a person found himself infected with the disease, as if a sentence of death had passed upon him, his spirits failed him, he fell into melancholy and despair, thought of nothing but death, and so gave up the ghost.
I think that's very significant. We are ALL under sentence of death because we all die. That's one of the Vatican Sayings (which one?) that we have all drunk a draft of death (?)
So here he is condemning as wretched and deplorable the "giving up" that comes from realizing that we are going to die. In contrast, we all should live with that realization every day, and not turn nihilist, but take that much more pleasure in the time we have because of it!
In fact at the moment I would tentatively see this as one of the most important aspects of the whole plague sequence: we are all under a death sentence from the moment we are born, but that doesn't mean we should become nihilists and run about crazily and fail to be friendly with each other and help our friends through hard times. We treasure life while we have it, and the loss of it may be inevitable but no reason to give up what we can gain while we have it.