“Now this is the strange thing about these germs. There were always new ones coming to live in men’s bodies. Long and long and long ago, when there were only a few men in the world, there were few diseases. But as men increased and lived closely together in great cities and civilizations, new diseases arose, new kinds of germs entered their bodies. Thus were countless millions and billions of human beings killed. And the more thickly men packed together, the more terrible were the new diseases that came to be. Long before my time, in the middle ages.