Today I'm just going to refer you to something that I found very interesting. It's a post by William Jones about Biopunk. Biopunk appears to be a particular type of post-apocalyptic fiction, in which the apocalypse is brought on by some kind of biological pandemic. I suggested in my comments to William's post that The Stand by Stephen King might be an early example of this relatively unknown genre.
I also suggested that post-apocalyptic fiction in general arises out of the fact that humans, alone of all the species on earth, know absolutely from early on that we will die, and because we also have a strong suspicion that our civilization itself and the whole human race will eventually die as well. In fact, I think this is a driving force behind much of what humans do, but that probably needs to be expanded in a longer post.