It is essential, however, that the individual not die before the new generation has been produced; at least, not in so many cases as to ensure the population dwindling to extinction. The human species cannot have the relative immunity to harm from individual death possessed by the small and fecund species. Human beings are comparatively large, long-lived, and slow to reproduce, so that too rapid individual death holds within it the specter of catastrophe. The rapid death of unusually high numbers of human beings through disease can seriously dent the human population. Carried to an extreme, it is not too hard to imagine it wiping out the human species. Most dangerous in this respect is that class of malfunction referred to as “infectious disease.” There are many disorders that affect a particular human being for one reason or another and may kill him or her, too, but which will not, in itself, offer a danger to the species, because it is strictly confined to the suffering individual. Where, however, a disease can, in some way travel from one human being to another, and where its occurrence in a single individual may lead to the death of not that one alone but of millions of others as well, then there is the possibility of catastrophe. And indeed, infectious disease has come closer to destroying the human species in historic times than have the depredations of any animals. Although infectious disease, even at its worst, has never yet actually put an end to human beings as a living species (obviously), it can seriously damage a civilization and change the course of history. It has, in fact, done so not once, but many times. What’s more, the situation has perhaps grown worse with the coming of civilization. Civilization has meant the development and growth of cities and the crowding of people into close quarters. Just as fire can spread much more rapidly from tree to tree in a dense forest than in isolated stands, so can infectious disease spread more quickly in crowded quarters than in sparse settlements. To mention a few notorious cases in history: In 431 B.C., Athens and its allies went to war with Sparta and its allies. It was a twenty-seven-year war that ruined Athens and, to a considerable extent, all of Greece. Since Sparta controlled the land, the entire Athenian population crowded into the walled city of Athens. There they were safe and could be provisioned by sea, which was controlled by the Athenian navy. Athens would very likely have won a war of attrition before long and Greece might have avoided ruin, but for disease.
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