When Huck Finn’s drunken “Pap” fell over a tub of salt pork and barked both shins, he fetched the tub a rattling kick. But it warn’t good judgment because that was the boot that had a couple of his toes leaking out the front end . . . and the cussing he done then laid over everything he had ever done previous.

Pap acted as if the tub wanted to hurt him, as if kicking and curs- ing it could deter future harm to his shins. But the kicking and cussing were wasted effort. The tub was not a rival trying to steal Pap’s mate, a predator trying to catch him, or even a microorganism stealthily trying to devour his tissues. It was merely inanimate wood.

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In discussing such things as tubs of salt pork as sources of injury, we leave behind the conflicting interests, strategies, and arms races that complicate contests between living opponents. The problems associated with injuries are conceptually simpler than those of infec- tious diseases, but there is complexity aplenty. Some dangers, like being struck by a meteorite, have always been so rare and unpre- dictable that we have no evolved defenses and can repair the damage only by using general-purpose mechanisms. Others, like exposure to high levels of gamma rays, are so new that we have not had time to evolve adequate defenses. But some dangers, like drowning or attack by predators, have happened often enough in evolutionary history that we have evolved ways to avoid them. This chapter is about the ways we avoid, escape, and repair damage from sources of injury such as mechanical trauma, radiation, burning, and freezing. It is also about why these adaptations do not always work as well as we might wish.