A Short History of Nearly Everything-第67章

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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　all　strains　of　staphylococcus　bacteria；　to　suchan　extent　that　by　the　early　1960s　the　u。s。　surgeon　general；　william　stewart；　felt　confidentenough　to　declare：　“the　time　has　e　to　close　the　book　on　infectious　diseases。　we　havebasically　wiped　out　infection　in　the　united　states。”　even　as　he　spoke；　however；　some　90percent　of　those　strains　were　in　the　process　of　developing　immunity　to　penicillin。　soon　one　ofthese　new　strains；　called　methicillin…resistant　staphylococcus　aureus；　began　to　show　up　inhospitals。　only　one　type　of　antibiotic；　vanycin；　remained　effective　against　it；　but　in　1997a　hospital　in　tokyo　reported　the　appearance　of　a　strain　that　could　resist　even　that。　withinmonths　it　had　spread　to　six　other　japanese　hospitals。　all　over；　the　microbes　are　beginning　towin　the　war　again：　in　u。s。　hospitals　alone；　some　fourteen　thousand　people　a　year　die　frominfections　they　pick　up　there。　as　james　surowiecki　has　noted；　given　a　choice　betweendeveloping　antibiotics　that　people　will　take　every　day　for　two　weeks　or　antidepressants　thatpeople　will　take　every　day　forever；　drug　panies　not　surprisingly　opt　for　the　latter。

although　a　few　antibiotics　have　been　toughened　up　a　bit；　the　pharmaceutical　industry　hasn’tgiven　us　an　entirely　new　antibiotic　since　the　1970s。

our　carelessness　is　all　the　more　alarming　since　the　discovery　that　many　other　ailments　maybe　bacterial　in　origin。　the　process　of　discovery　began　in　1983　when　barry　marshall；　a　doctorin　perth；　western　australia；　found　that　many　stomach　cancers　and　most　stomach　ulcers　arecaused　by　a　bacterium　called　helicobacter　pylori。　even　though　his　findings　were　easily　tested；the　notion　was　so　radical　that　more　than　a　decade　would　pass　before　they　were　generallyaccepted。　america’s　national　institutes　of　health；　for　instance；　didn’t　officially　endorse　theidea　until　1994。　“hundreds；　even　thousands　of　people　must　have　died　from　ulcers　whowouldn’t　have；”　marshall　told　a　reporter　from　forbes　in　1999。

since　then　further　research　has　shown　that　there　is　or　may　well　be　a　bacterial　ponent　inall　kinds　of　other　disorders—heart　disease；　asthma；　arthritis；　multiple　sclerosis；　several　typesof　mental　disorders；　many　cancers；　even；　it　has　been　suggested　（inscience　no　less）；　obesity。

the　day　may　not　be　far　off　when　we　desperately　require　an　effective　antibiotic　and　haven’tgot　one　to　call　on。

it　may　e　as　a　slight　fort　to　know　that　bacteria　can　themselves　get　sick。　they　aresometimes　infected　by　bacteriophages　（or　simply　phages）；　a　type　of　virus。　a　virus　is　a　strangeand　unlovely　entity—“a　piece　of　nucleic　acid　surrounded　by　bad　news”　in　the　memorablephrase　of　the　nobel　laureate　peter　medawar。　smaller　and　simpler　than　bacteria；　viruses　aren’tthemselves　alive。　in　isolation　they　are　inert　and　harmless。　but　introduce　them　into　a　suitablehost　and　they　burst　into　busyness—into　life。　about　five　thousand　types　of　virus　are　known；and　between　them　they　afflict　us　with　many　hundreds　of　diseases；　ranging　from　the　flu　andmon　cold　to　those　that　are　most