You know, reading through all of these "reasons," it came to me in two simple words:
Population density
150 years ago, folks had to figure it out or die. If their mistakes didn't kill them, they damn well had better learned from them. Of course, there were cities then too and within the cities things were going to hell, but....
Look, put too many rats or damn near ANY other animal in too close proximity and they're going to kill each other. If they don't, nature will step in. I don't know if it'll happen in our lifetime, but either animal nature (I don't call it human nature, because I don't think we have the corner on the market) OR nature itself will find a "cure."
