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according to Black； also occurred at the same time； no doubt triggered by the earthquake。〃
〃So you think an earthquake killed all those people？〃 Nora asked。
〃No； no。 It was just a temblor。 But that rockfall； and the collapse of some buildings； was enough to raise a large cloud of dust in the valley。〃
〃Very interesting；〃 Smithback said。 〃But what does a seven…century…old dustcloud have to do with Holroyd's death？〃
Aragon gave a wan smile。 〃A great deal； as it turns out。 Because the dust within Quivira is riddled with Coccidioides immitis。 It's a microscopic fungal spore that lives in soil。 It's usually associated with very dry； often remote desert areas； so people don't e into contact with it much。 Which is a very good thing。 It's the cause of a deadly disease known as coccidioidomycosis。 Or； as you might know it； valley fever。〃
Nora frowned。 〃Valley fever？〃
〃Wait a minute；〃 Smithback interjected。 〃Wasn't that the disease that killed a bunch of people in California？〃
Aragon nodded。 〃Valley fever； or San Joaquin fever； named after a town in California。 There was an earthquake in the desert near San Joaquin many years ago。 That quake triggered a small landslide that raised a cloud of dust； which rolled over the town。 Hundreds became ill and twenty died； infected with coccidioidomycosis。 Scientists came to call this type of deadly dustcloud a 'tectonic fungal cloud。'〃 He frowned。 〃Only the fungus here in Quivira is a far more virulent strain。 In concentrated form； it kills in hours or days； not weeks。 You see； to get sick you must inhale the spores…either through dust； or through 。 。 。 other means。 Mere exposure to a sick person is not enough。〃
He wiped his face again。 〃At first； Holroyd's symptoms were baffling to me。 They did not seem to be from any infectious agent I knew of。 Certainly he died too quickly for any of the more likely suspects。 And then I remembered that rust…colored powder from the royal burial。〃
He looked at Nora。 〃I told you about my discoveries with the bones。 But do you recall those two pots； full of reddish dust？ You thought they might be a kind of red ochre。 I never told you that the dust turned out to be dried； ground…up human flesh and bone。〃
〃Why didn't you tell us？〃 Nora cried。
〃Let's just say you were preoccupied with other things。 And I wanted to understand it myself before I dangled yet another mystery in front of you。 In any case； while puzzling over Holroyd's death； I remembered that reddish dust。 And then I realized exactly what it was。 It is a substance known to certain southwestern Indian tribes as 'corpse powder。'〃
Nora glanced at Smithback and saw her own horror reflected in his eyes。
〃It's used by witches to kill their intended victims；〃 Aragon continued。 〃Corpse powder is still known among some Indian groups today。〃
〃I know；〃 Nora whispered。 She could almost see Beiyoodzin's drawn face in the starlight； telling them of the wolfskin runners。
〃When I examined this powder under the microscope； I found it absolutely packed with Coccidioides immitis。 It is； quite literally； corpse powder that really kills。〃
〃And you think Holroyd was murdered with it？〃
〃Given the huge dose he must have received to die so quickly； I would say yes。 Although his illness was surely made worse by constant exposure to dust。 He did quite a lot of digging in the rear of the ruin in the days before his death。 The fact is； we've all been exposed to it。〃
〃I did my share of digging；〃 Smithback said； his voice a little shaky。 〃How much longer before we get sick； too？〃
〃I don't know。 A lot depends on the health of our immune systems； and on the degree of exposure。 I believe the fungus is much more concentrated in the rear of the city。 But regardless； it's vital that we get out of here and get treatment as soon as possible。〃
〃So there's a cure？〃 Smithback asked。
〃Yes。 Ketoconazole； or in advanced cases where the fungus has invaded the central nervous system； amphotericin B injected directly into the cerebrospinal fluid。 The ironic thing is； ampho is a mon antibiotic。 I almost brought some along。〃
〃How sure are you about this？〃 asked Nora。
〃As sure I can be without more equipment。 I'd need a better microscope to be absolutely sure； because in tissue the spherules are only about fifty microns in diameter。 But nothing else explains the onset of symptoms： the cyanosis； dyspnea； the mucopurulent sputum 。 。 。 the sudden death。 And the simple test I just performed on Peter's lung tissue confirmed the presence of coccidioidin antibodies。〃 He sighed。 〃It's only in the last day or so that I began putting this together。 Late yesterday evening； I spent some time in the ruin； and found other examples of corpse powder stored in pots； as well as various odd types of tools。 From this； and from all the trashed bones in the Crawlspace； it became quite clear that the inhabitants of Quivira were actually manufacturing corpse powder。 As a result； the whole city is contaminated with it。 The entire subsoil of the ruin is full of the spores； its density increasing toward the back。 That puts the greatest concentration in the Crawlspace； and especially in the cavern of the Sun Kiva that Black discovered。〃
He paused。 〃I told you my theory that this city was not really Anasazi after all。 It was Aztecan in origin。 These people brought human sacrifice and witchcraft to the Anasazi。 It's my belief that they are the marauders； the conquerors； who caused the collapse of Anasazi civilization and the abandonment of the Colorado Plateau。 They are the mysterious enemies of the Anasazi that archaeologists have sought all these years。 These enemies did not kill and exert control through open warfare； which is why we've never found the evidence of violence。 Their means of conquest and control were more subtle。 Witchcraft and the use of corpse powder。 Which leaves little or no trace。〃
His voice fell。 〃When I first analyzed that burial cyst Sloane uncovered； I felt it to be a result of cannibalism。 The marks on the bones seemed to point to that。 In fact； Black's protests to the contrary； it was the obvious deduction to make： Anasazi cannibalism is currently a hot； if controversial； theory。 But I no longer think cannibalism is at the bottom of all this。 I now believe those marks on the bones tell an even more terrible tale。〃
He looked at Nora with haunted eyes。 〃I believe the priests of the city were infecting prisoners or slaves with the disease； waiting for them to die； and then processing their bodies to make corpse powder。 The trash from that terrible operation lies in the back of the cave。 With the powder； these conquerors could maintain their rule through ritual and terror。 But in the end； the fungus turned on them。 The mild earthquake that damaged the towers and caused the landslide must have raised a tectonic fungal cloud in the valley here； just like in San Joaquin。 Except that here； in the confined space of the canyon； the dustcloud had no place to go。 It filled the alcove； enshrouded the city of Quivira。 All those skeletons； thrown atop the broken bodies in the back of the cave； were its priestly Aztec victims。〃
Aragon stopped speaking and looked away from Nora。 His face； she thought； had never looked so drawn； so exhausted。
〃Now； it's time for me to tell you something；〃 Nora replied slowly。 〃Modern…day witches may be the ones trying to drive us out of the valley。〃 She briefly told Aragon about the attack in the ranch house and the more recent conversation with Beiyoodzin。 〃They followed us out here；〃 she concluded。 〃And now that they've found the site； they're trying to drive us away so they can loot it for themselves。〃
Aragon thought for a moment； then shook his head。 〃No；〃 he said。 〃No； I don't think they're here to loot the city。〃
〃What are you talking about？〃 Smithback interjected。 〃Why else would they be trying to drive us away？〃
〃Oh； I don't dispute they're trying to drive us away。 But it's not to loot the city。〃 He glanced once again at Nora。 〃You've been assuming all along that these skinwalkers were trying to find the city。 What if they were actually trying to protect it？〃
〃I don't…〃 Smithback began。
〃Just a minute；〃 Nora brok