identity and expel them from their entry portal. If they have breached this defense line, it can poke holes in them, poison them, starve them, do whatever is necessary to kill them. And if all this does not work, it can wall them off so that they cannot reproduce and spread. If they have done damage, it can repair it. If the damage can’t be repaired immediately, it can compensate forit in some way. Some of this damage and the resulting impairment benefit neither the host nor the pathogen. They are, like the aging bomb craters on the coast of France, just incidental relics of an old battle.
The pathogens will not, of course, give up readily. Our bodies are, after all, their homes and dinners. We understandably tend to see bacteria and viruses as evils, but how anthropocentric this is! Our defenses attempt to prevent the poor streptococcus from getting even a microgram of our body tissues, but if it cannot find a way around our defenses, it will die. So, for each of our defenses, pathogens have evolved counterdefenses. They find ways to get transmitted to us and ways to breach our walls. Once inside, they hide from our sentries, attack our defenses, use our nutrients to make copies of themselves, and find ways to get those copies out of the body and to new victims, often by turning our own defenses to their own advantage. Before describing the clever stratagems used by pathogens to elude our defenses, we will discuss the defenses in more detail.
H YGIENE
T he best defense is avoidance of danger; proper hygiene can prevent a pathogen from gaining that first toehold. Instinctively slapping at a mosquito is not just an attempt to spare oneself the minor annoyance of a mosquito bite. It may also prevent a long list of serious insect-borne diseases, of which malaria is the best known. Is the itch of a mosquito bite just part of the insect’s nastiness? It may be merely an accidental result of the chemicals the mosquito uses to ensure that our blood flows freely, but it may also be our adaptation for avoiding future bites. Imagine what would happen to a person who did not mind being bitten by mosquitoes. And imagine how successful a mosquito could be if its biting were not noticeable!
Our tendencies to avoid contact with people who may be infectious may have the same significance. Likewise, an instinctive disgust motivates us to avoid feces, vomit, and other sources of contagion. Our tendency to defecate away from others may prevent the infection of close associates, and social pressures to conform to such practices may protect us from infection by others. The best defenseagainst infection is avoidance of pathogens, and natural selection has shaped many mechanisms to help us keep our distance.
T HE S KIN
O ur skin is like the wall around an ancient city, a formidable protective barrier. It not only prevents the entry of parasites but also protects against injury by mechanical, thermal, and chemical forces. Unlike induced defenses such as fever, which are aroused only when a particular danger threatens, the skin is constantly present, always on guard. It is tough and much more resistant to puncture and abrasion than the internal tissues it protects. Minor infections here and there are harmless because the skin is constantly being sloughed off the top and renewed from below. An ink stain on the fingers will be gone in a few days, not because the ink has been absorbed or chemically altered but because the stained cells are replaced by others rising from below. Fungal growths or other potential pathogens in surface cells are constantly cast off by this rapid replacement of the epidermis. Sycamores and shagbark hickory trees seem to use the same strategy.
Not only is the skin a good defensive armor in general, it is also good in particular. Those parts of the body that are most in need of armor, such as the soles of the feet, have thicker and tougher skin right from birth. Any particular patch of skin that is subjected to repeated
The pathogens will not, of course, give up readily. Our bodies are, after all, their homes and dinners. We understandably tend to see bacteria and viruses as evils, but how anthropocentric this is! Our defenses attempt to prevent the poor streptococcus from getting even a microgram of our body tissues, but if it cannot find a way around our defenses, it will die. So, for each of our defenses, pathogens have evolved counterdefenses. They find ways to get transmitted to us and ways to breach our walls. Once inside, they hide from our sentries, attack our defenses, use our nutrients to make copies of themselves, and find ways to get those copies out of the body and to new victims, often by turning our own defenses to their own advantage. Before describing the clever stratagems used by pathogens to elude our defenses, we will discuss the defenses in more detail.
H YGIENE
T he best defense is avoidance of danger; proper hygiene can prevent a pathogen from gaining that first toehold. Instinctively slapping at a mosquito is not just an attempt to spare oneself the minor annoyance of a mosquito bite. It may also prevent a long list of serious insect-borne diseases, of which malaria is the best known. Is the itch of a mosquito bite just part of the insect’s nastiness? It may be merely an accidental result of the chemicals the mosquito uses to ensure that our blood flows freely, but it may also be our adaptation for avoiding future bites. Imagine what would happen to a person who did not mind being bitten by mosquitoes. And imagine how successful a mosquito could be if its biting were not noticeable!
Our tendencies to avoid contact with people who may be infectious may have the same significance. Likewise, an instinctive disgust motivates us to avoid feces, vomit, and other sources of contagion. Our tendency to defecate away from others may prevent the infection of close associates, and social pressures to conform to such practices may protect us from infection by others. The best defenseagainst infection is avoidance of pathogens, and natural selection has shaped many mechanisms to help us keep our distance.
T HE S KIN
O ur skin is like the wall around an ancient city, a formidable protective barrier. It not only prevents the entry of parasites but also protects against injury by mechanical, thermal, and chemical forces. Unlike induced defenses such as fever, which are aroused only when a particular danger threatens, the skin is constantly present, always on guard. It is tough and much more resistant to puncture and abrasion than the internal tissues it protects. Minor infections here and there are harmless because the skin is constantly being sloughed off the top and renewed from below. An ink stain on the fingers will be gone in a few days, not because the ink has been absorbed or chemically altered but because the stained cells are replaced by others rising from below. Fungal growths or other potential pathogens in surface cells are constantly cast off by this rapid replacement of the epidermis. Sycamores and shagbark hickory trees seem to use the same strategy.
Not only is the skin a good defensive armor in general, it is also good in particular. Those parts of the body that are most in need of armor, such as the soles of the feet, have thicker and tougher skin right from birth. Any particular patch of skin that is subjected to repeated
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