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e a host body； so howcould it appear in madrid； bombay； and philadelphia all in the same week？
the probable answer is that it was incubated and spread by people who had only slightsymptoms or none at all。 even in normal outbreaks； about 10 percent of people have the flubut are unaware of it because they experience no ill effects。 and because they remain incirculation they tend to be the great spreaders of the disease。
that would account for the 1918 outbreak鈥檚 widespread distribution； but it still doesn鈥檛explain how it managed to lay low for several months before erupting so explosively at moreor less the same time all over。 even more mysterious is that it was primarily devastating topeople in the prime of life。 flu normally is hardest on infants and the elderly； but in the 1918outbreak deaths were overwhelmingly among people in their twenties and thirties。 olderpeople may have benefited from resistance gained from an earlier exposure to the same strain；but why the very young were similarly spared is unknown。 the greatest mystery of all is whythe 1918 flu was so ferociously deadly when most flus are not。 we still have no idea。
from time to time certain strains of virus return。 a disagreeable russian virus known ash1n1 caused severe outbreaks over wide areas in 1933； then again in the 1950s； and yet againin the 1970s。 where it went in the meantime each time is uncertain。 one suggestion is thatviruses hide out unnoticed in populations of wild animals before trying their hand at a newgeneration of humans。 no one can rule out the possibility that the great swine flu epidemicmight once again rear its head。
and if it doesn鈥檛； others well might。 new and frightening viruses crop up all the time。
ebola； lassa； and marburg fevers all have tended to flare up and die down again； but no onecan say that they aren鈥檛 quietly mutating away somewhere； or simply awaiting the rightopportunity to burst forth in a catastrophic manner。 it is now apparent that aids has beenamong us much longer than anyone originally suspected。 researchers at the manchesterroyal infirmary in england discovered that a sailor who had died of mysterious； untreatablecauses in 1959 in fact had aids。 but for whatever reasons the disease remained generallyquiescent for another twenty years。
the miracle is that other such diseases haven鈥檛 gone rampant。 lassa fever； which wasn鈥檛first detected until 1969； in west africa； is extremely virulent and little understood。 in 1969； adoctor at a yale university lab in new haven； connecticut； who was studying lassa fevercame down with it。 he survived； but； more alarmingly； a technician in a nearby lab； with nodirect exposure； also contracted the disease and died。
happily the outbreak stopped there； but we can鈥檛 count on such good fortune always。 ourlifestyles invite epidemics。 air travel makes it possible to spread infectious agents across theplanet with amazing ease。 an ebola virus could begin the day in； say； benin； and finish it innew york or hamburg or nairobi； or all three。 it means also that medical authoritiesincreasingly need to be acquainted with pretty much every malady that exists everywhere； butof course they are not。 in 1990； a nigerian living in chicago was exposed to lassa fever on avisit to his homeland； but didn鈥檛 develop symptoms until he had returned to the united states。
he died in a chicago hospital without diagnosis and without anyone taking any specialprecautions in treating him； unaware that he had one of the most lethal and infectious diseaseson the planet。 miraculously； no one else was infected。 we may not be so lucky next time。
and on that sobering note； it鈥檚 time to return to the world of the visibly living。
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21 LIFE GOES ON
灏彛昏…t…xt澶ｅ爞
it isn鈥檛 easy to bee a fossil。 the fate of nearly all living organisms鈥攐ver 99。9percent of them鈥攊s to post down to nothingness。 when your spark is gone； everymolecule you own will be nibbled off you or sluiced away to be put to use in some othersystem。 that鈥檚 just the way it is。 even if you make it into the small pool of organisms； the lessthan 0。1 percent； that don鈥檛 get devoured； the chances of being fossilized are very small。
in order to bee a fossil； several things must happen。 first； you must die in the rightplace。 only about 15 percent of rocks can preserve fossils； so it鈥檚 no good keeling over on afuture site of granite。 in practical terms the deceased must bee buried in sediment； whereit can leave an impression； like a leaf in wet mud； or depose without exposure to oxygen；permitting the molecules in its bones and hard parts （and very occasionally softer parts） to bereplaced by dissolved minerals； creating a petrified copy of the original。 then as thesediments in which the fossil lies are carelessly pressed and folded and pushed about byearth鈥檚 processes； the fossil must somehow maintain an identifiable shape。 finally； but aboveall； after tens of millions or perhaps hundreds of millions of years hidden away； it must befound and recognized as something worth keeping。
only about one bone in a billion； it is thought； ever bees fossilized。 if that is so； itmeans that the plete fossil legacy of all the americans alive today鈥攖hat鈥檚 270 millionpeople with 206 bones each鈥攚ill only be about fifty bones； one quarter of a pleteskeleton。 that鈥檚 not to say of course that any of these bones will actually be found。 bearing inmind that they can be buried anywhere within an area of slightly over 3。6 million squaremiles； little of which will ever be turned over； much less examined； it would be something ofa miracle if they were。 fossils are in every sense vanishingly rare。 most of what has lived onearth has left behind no record at all。 it has been estimated that less than one species in tenthousand has made it into the fossil record。 that in itself is a stunningly infinitesimalproportion。 however； if you accept the mon estimate that the earth has produced 30billion species of creature in its time and richard leakey and roger lewin鈥檚 statement （inthe sixth extinction ） that there are 250；000 species of creature in the fossil record； thatreduces the proportion to just one in 120；000。