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ch is as different from its neighbors as animals are from plants。 ifwoese’s new arrangement teaches us anything it is that life really is various and that most ofthat variety is small； unicellular； and unfamiliar。 it is a natural human impulse to think ofevolution as a long chain of improvements； of a never…ending advance toward largeness andplexity—in a word； toward us。 we flatter ourselves。 most of the real diversity inevolution has been small…scale。 we large things are just flukes—an interesting side branch。 ofthe twenty…three main divisions of life； only three—plants； animals； and fungi—are largeenough to be seen by the human eye； and even they contain species that are microscopic。
indeed； according to woese； if you totaled up all the biomass of the planet—every living thing； plants included—microbes would account for at least 80 percent of all there is； perhapsmore。 the world belongs to the very small—and it has for a very long time。
so why； you are bound to ask at some point in your life； do microbes so often want to hurtus？ what possible satisfaction could there be to a microbe in having us grow feverish orchilled； or disfigured with sores； or above all expire？ a dead host； after all； is hardly going toprovide long…term hospitality。
to begin with； it is worth remembering that most microorganisms are neutral or evenbeneficial to human well…being。 the most rampantly infectious organism on earth； abacterium called wolbachia； doesn’t hurt humans at all—or； e to that； any othervertebrates—but if you are a shrimp or worm or fruit fly； it can make you wish you had neverbeen born。 altogether； only about one microbe in a thousand is a pathogen for humans；according to national geographic —though； knowing what some of them can do； we couldbe forgiven for thinking that that is quite enough。 even if mostly benign； microbes are still thenumber…three killer in the western world； and even many less lethal ones of course make usdeeply rue their existence。
making a host unwell has certain benefits for the microbe。 the symptoms of an illnessoften help to spread the disease。 vomiting； sneezing； and diarrhea are excellent methods ofgetting out of one host and into position for another。 the most effective strategy of all is toenlist the help of a mobile third party。 infectious organisms love mosquitoes because themosquito’s sting delivers them directly to a bloodstream where they can get straight to workbefore the victim’s defense mechanisms can figure out what’s hit them。 this is why so manygrade…a diseases—malaria； yellow fever； dengue fever； encephalitis； and a hundred or soother less celebrated but often rapacious maladies—begin with a mosquito bite。 it is afortunate fluke for us that hiv； the aids agent； isn’t among them—at least not yet。 any hivthe mosquito sucks up on its travels is dissolved by the mosquito’s own metabolism。 whenthe day es that the virus mutates its way around this； we may be in real trouble。
it is a mistake； however； to consider the matter too carefully from the position of logicbecause microorganisms clearly are not calculating entities。 they don’t care what they do toyou any more than you care what distress you cause when you slaughter them by the millionswith a soapy shower or a swipe of deodorant。 the only time your continuing well…being is ofconsequence to a pathogen is when it kills you too well。 if they eliminate you before they canmove on； then they may well die out themselves。 this in fact sometimes happens。 history；jared diamond notes； is full of diseases that “once caused terrifying epidemics and thendisappeared as mysteriously as they had e。” he cites the robust but mercifully transientenglish sweating sickness； which raged from 1485 to 1552； killing tens of thousands as itwent； before burning itself out。 too much efficiency is not a good thing for any infectiousorganism。
a great deal of sickness arises not because of what the organism has done to you but whatyour body is trying to do to the organism。 in its quest to rid the body of pathogens； theimmune system sometimes destroys cells or damages critical tissues； so often when you areunwell what you are feeling is not the pathogens but your own immune responses。 anyway；getting sick is a sensible response to infection。 sick people retire to their beds and thus areless of a threat to the wider munity。 resting also frees more of the body’s resources toattend to the infection。
because there are so many things out there with the potential to hurt you； your body holdslots of different varieties of defensive white cells—some ten million types in all； eachdesigned to identify and destroy a particular sort of invader。 it would be impossibly inefficientto maintain ten million separate standing armies； so each variety of white cell keeps only afew scouts on active duty。 when an infectious agent—what’s known as an antigen—invades；relevant scouts identify the attacker and put out a call for reinforcements of the right type。
while your body is manufacturing these forces； you are likely to feel wretched。 the onset ofrecovery begins when the troops finally swing into action。
white cells are merciless and will hunt down and kill every last pathogen they can find。 toavoid extinction； attackers have evolved two elemental strategies。 either they strike quicklyand move on to a new host； as with mon infectious illnesses like flu； or they disguisethemselves so that the white cells fail to spot them； as with hiv； the virus responsible foraids； which can sit harmlessly and unnoticed in the nuclei of cells for years before springinginto action。
one of the odder aspects of infection is that microbes that normally do no harm at allsometimes get into the wrong parts of the body and “go kind of crazy；” in the words of dr。
bryan marsh； an infectious diseases specialist at dartmouth–hitchcock medical center inlebanon； new hamphire。 “it happens all the time with car accidents when people sufferinternal injuries。 microbes that are normally benign in the gut get into other parts of thebody—the bloodstream； for instance—and cause terrible havoc。”
the scariest； most out…of…control bacterial disorder of the moment is a disease callednecrotizing fasciitis in which bacteria essentially eat the victim from the inside out； devouringinternal tissue and leaving behind a pulpy； noxious residue。 patients often e in withparatively mild plaints—a skin rash and fever typically—but then dramaticallydeteriorate。 when they are opened up it is often found that they are simply being consumed。
the only treatment is what is known as “radical excisional surgery”—cutting out every bit ofinfected area。 seventy percent of victims die； many of the rest are left terribly disfigured。 thesource of the infection is a mundane family of bacteria called group a streptococcus； whichnormally do no more than cause strep throat。 very occasionally； for reasons unknown； someof these bacteria get through the lining of the throat and into the body proper； where theywreak the most devastating havoc。 they are pletely resistant to antibiotics。 about athousand cases a year occur in the united states； and no one can say that it won’t get worse。
precisely the same thing happens with meningitis。 at least 10 percent of young adults； andperhaps 30 percent of teenagers； carry the deadly meningococcal bacterium； but it lives quiteharmlessly in the throat。 just occasionally—in about one young person in a hundredthousand—it gets into the bloodstream and makes them very ill indeed。 in the worst cases；death can e in twelve hours。 that’s shockingly quick。 “you can have a person who’s inperfect health at breakfast and dead by evening；” says marsh。
we would have much more success with bacteria if we weren’t so profligate with our bestweapon against them： antibiotics。 remarkably； by one estimate some 70 percent of theantibiotics used in the developed world are given to farm animals； often routinely in stockfeed； simply to promote growth or as a precaution against infection。 such applications givebacteria every opportunity to evolve a resistance to them。 it is an opportunity that they haveenthusiastically seized。
in 1952； penicillin was fully effective against a