12 May

Astrologers blamed the Black Death on a malign conjunction  of Saturn, Jupiter,  and  Mars.  Epidemiologists  have traced  the cause of epidemic plague to a catastrophic conjunction  of Yersinia pestis, fleas, and rats. A brief overview of the complex ecological relationships of microbes, fleas, rodents,  and human  beings will help us understand the medieval pandemics, the waves of plague that continued  well into the seventeenth century, and the status of plague today. Studying the components  of this web of relationships should help dispel the notion  that  discovering the ‘‘cause’’ of  epidemic  disease  is a  simple  matter  of  finding  a  specific microbial  agent.  Even if a specific pathogen  can be linked to epidemic disease, that microbe is only one strand in the complex web of life, along with fleas, mosquitoes,  lice, ticks, wild animals,  domesticated animals, and  human  beings. Moreover,  the relationship  between human  beings and  epidemic disease is affected  by many  factors:  biological,  climatic, social,  cultural,  political,  economic,  and  so  forth.  The  magnitude  of the  plague  pandemics  provides  a  striking  demonstration of just  how powerful a force disease can be in history. Such reminders  are essential now that  molecular biologists are able to identify, isolate, and manipu- late  the  genetic  factors  responsible  for  the  awesome  virulence  of the microbes that  cause bubonic  plague and other epidemic diseases.

Bacteriologists    and   epidemiologists   have   examined   historical accounts  of plague and laboratory studies of recent outbreaks in order to  determine  the  natural history  of plague  and  its clinical pattern of signs and symptoms.  Attempts  to compare  modern  clinical and labora- tory descriptions  of bubonic plague with eyewitness accounts of ancient and  medieval  epidemics  reveal  the  difficulties  inherent   in  attaching modern  diagnoses  to  ancient  diseases.  Historical  accounts  of  devas- tating epidemics are often vague, confusing, and highly stylized in terms of the signs and symptoms that physicians and laymen considered significant.  Fourteenth-century accounts  of the  Black  Death  describe horrific   symptoms   that   included   painfully   swollen   lymph   nodes, gangrenous organs, bleeding from the nose, bloody sputum, and hemorrhaging blood  vessels, which caused splotches  and discoloration of the skin.

To add  to the confusion,  bubonic  plague  provides  an interesting example of the way in which a specific microbe can cause different clini- cal  patterns. In  this  case,  the  major  forms  of  illness  are  known  as bubonic plague  and  pneumonic plague;  a  rare  form  of  the  disease  is known  as septicemic plague.  In the absence of appropriate antibiotics, the mortality rate for bubonic plague may exceed 50 percent; pneumonic plague  and  septicemic plague  are almost  invariably  fatal.  Even today, despite  streptomycin, tetracycline,  and  chloramphenicol, many  plague victims succumb to the disease.

If Y. pestis, the plague bacillus, enters the body via the bite of an infected flea, the disease follows the pattern known as bubonic. After an incubation  period  that  may last for two to six days, during  which the bacteria multiply in the lymph nodes, victims suddenly experience fever, headache,  pains in the chest, coughing, difficulty in breathing,  vomiting of blood,  and dark  splotches on the skin. The most characteristic  signs of bubonic  plague are the painful swellings called buboes that appear  in the  lymph  nodes,   usually  in  the  groin,   armpits,   and   neck.  Other

Religious depiction

Religious depiction of proper responses to the plague.

symptoms  include  restlessness,  anxiety,  mental  confusion,  hallucina- tions,   and   coma.   Certain   bacterial   proteins   inhibit   the   immune responses  that  would  otherwise  block  the multiplication and  dissemi- nation  of the  bacteria.  Plague  bacteria  also  release a toxin  that  may result  in  shock,  circulatory   collapse,  widespread  organ  failure,  and, finally, death. In septicemic plague, the bacteria spread rapidly through- out the bloodstream, damaging  internal organs and blood vessels, lead- ing to  gangrene,  internal  hemorrhaging, bleeding  from  the  nose  and ears, delirium, or coma. Death  occurs within one to three days, without the appearance of buboes.

Spread directly from person to person by droplets  of saliva, pneu- monic plague is highly contagious and exceptionally lethal. Just what cir- cumstances lead to widespread transformation of bubonic plague to the pneumonic  form  is uncertain.  When large numbers  of bacteria  spread to  the  lungs of patients  with  bubonic  plague,  resulting  in pulmonary abscesses, fits of coughing and sneezing release droplets  of sputum  con- taining hordes of bacteria.  When inhaled into the respiratory system of new hosts, plague bacteria  multiply rapidly, resulting in the highly con- tagious condition  known as primary pneumonic  plague. The incubation period  for pneumonic  plague is usually only one to three days and the onset of symptoms  is very abrupt. Pain in the chest is accompanied by violent coughing that  brings up bloody sputum.  Neurological  disorders progress  rapidly  and  incapacitate the  victim.  Hemorrhages under  the skin produce  dark-purple blotches.  Coughing  and choking,  the patient finally  suffocates  and  dies. Patients  with  this  very lethal  form  of the disease experience high fever, chills, and a fatal pneumonia.

In 2001, researchers  succeeded in decoding  the genome and  plas-mids of a strain  of Y. pestis taken  from  a Colorado veterinarian who had  died of pneumonic  plague  in 1992. (The infection  was contracted from the sneeze of a cat.) On the basis of studies of the DNA  sequence, microbiologists  suggested that  Y. pestis probably  evolved about  20,000 years  ago  from  Yersinia pseudotuberculosis, a minor  human  intestinal pathogen.  Molecular  biologists  believe that  Y. pestis became a virulent pathogen  by acquiring  new genes, losing or silencing certain Y. pseudo- tuberculosis genes, and  establishing  a  remarkable pattern  of  chromo- somal rearrangements that make its genome unusually dynamic. By acquiring  genes from other  bacteria,  Y. pestis was able to colonize new environments. One of these genes apparently codes for an enzyme that allows the bacteria  to survive in the gut of a flea, which transforms the flea into a vector of the disease.

Some scientists warned that genomic sequence data could be used to create  more  deadly forms of pathogens  for use as biological  weapons, perhaps  more readily than genetic information could be used to develop preventive vaccines. Although  the strain of Y. pestis that was sequenced was already capable of causing death within 48 hours, experts in biologi- cal warfare point  out that  it might be possible to add genes that  would create variants that are resistant to antibiotics and any potential  vaccines.

Many aspects of the natural history of pandemic plague were finally clarified in the 1890s when successive outbreaks attacked  Canton, Hong Kong, Bombay, Java, Japan,  Asia Minor, South Africa, North  and South America,  Portugal, Austria,  and  parts  of Russia.  Some historical  epide- miologists  estimate  that  the  plague  epidemics of the  1890s killed more than  12 million people, but others  believe that  over 10 million people in India alone were killed by plague in the late nineteenth  and early twenti- eth centuries.  In 1894, Alexandre  Yersin (1863–1943) isolated the plague bacillus from the buboes of cadavers during an outbreak in Hong Kong. Using  the  sample  that   Yersin  sent  to  the  Pasteur   Institute   in  Paris, E´ mil  Roux  (1853–1933)  prepared   the  first  anti-plague   serum.  Yersin

called  the  microbe  Pasteurella   pestis,  in  honor   of  his  mentor,   Louis Pasteur.  Shibasaburo Kitasato (1852–1931), who  is best known  for  his studies  of  tetanus  and  diphtheria, independently   identified  the  plague bacillus  while studying  the  1894 Hong  Kong  plague  outbreak for  the Japanese  government.

In  1971, the  plague  bacillus  was renamed  Y. pestis, in honor  of Alexandre Yersin. At least three naturally  occurring varieties of Y. pestis are known today. All three varieties cause virulent infections in humans and most mammals. The microbe can remain viable for many months in the congenial microclimate of rodent warrens. Its life span in putrefying corpses is limited to a few days, but it may survive for years in frozen cadavers.  Thus,  local  outbreaks depend  on  the  state  of rodent  com- munities and the means used to dispose of the bodies of plague victims. During  the 1980s and 1990s, the World  Health  Organization recorded more than  18,000 cases of plague in 24 countries;  more than  half were in Africa.  In the United  States,  the disease was reported  in 13 states. By the  end  of the  1990s, epidemiologists  were warning  that  cases of plague were actually increasing throughout the world and that  the dis- ease should be classified as a re-emerging disease. Until  the late 1990s, the  plague  bacillus  was  universally  responsive  to  antibiotics.   Plague treated with antibiotics  had a mortality  rate of about 15 percent, in con- trast to estimates of 50 to 90 percent for untreated plague. Worse yet, a strain of plague bacilli recently discovered by researchers at the Pasteur Institute  of Madagascar is resistant  to streptomycin, chloramphenicol, tetracycline,  and  sulfonamides.   If  the  genes  for  antibiotic   resistance become widely disseminated among other strains of the bacillus, bubonic plague could again emerge as a very serious threat.

Although  Y. pestis can easily penetrate  mucous membranes, it can- not  enter  the  body  through   healthy,  unbroken skin.  Therefore,  the microbe  is generally dependent  on the flea to reach  new hosts.  In the 1890s, scientists reported  finding the plague bacillus in the stomach  of fleas taken  from  infected rats,  but  the ‘‘flea theory’’ was greeted  with such  skepticism  that  members  of  the  British  Plague  Commission   in Bombay  carried  out  experiments  to  prove  that  fleas did not transmit plague.  They ‘‘proved’’ their hypothesis  because they assumed  that  ‘‘a flea is a flea is a flea.’’ Further progress  in ‘‘fleaology’’ revealed  that all fleas are not created  equal.

Out of some two thousand different kinds of fleas, the black rat’s flea, Xenophylla cheopsis, deserves first place honors as the most efficient vector  of  plague,  but  at  least  eight  species of  fleas can  transmit  the microbe  to  humans.   Depending   on  host,  heat,  and  humidity,   fleas may  live for  only  a  few days  or  as long  as  a  year.  An  infected  flea actually  becomes  a victim of the  rapidly  multiplying  plague  bacillus. Eventually,  the flea’s stomach  becomes blocked  by a plug of bacteria. When  the  starving  flea bites  a new victim,  the  ingested  blood  comes in  contact   with  this  plug  and  mixes  with  the  bacteria.   Part  of  the ingested material, containing tens of thousands of bacilli, is regurgitated into  the  wound,  leading  to  multiplication of  plague  bacteria  in  the lymph glands nearest the bite. Fleas are usually fairly loyal to their pri- mary  host  species. Unfortunately, X. cheopsis finds human  beings an acceptable substitute for rats. Pulex irritans, the human flea, cannot approach the infective power of the rat flea, but under appropriate con- ditions  quantity   can  make  up  for  quality.  Despite  the  flea’s role  as ubiquitous  nuisance and vector of disease, Thomas Moffet (1553–1604), father  of Little  Miss Moffet,  noted  that,  in contrast  to being lousy, it was not a disgrace to have fleas.

Once the connection between rats and plague was elucidated, many authorities  believed that the black rat, Rattus rattus, was the only source of plague epidemics. However,  almost  two hundred  species of rodents have  been  identified  as possible  reservoirs  of plague.  The  concept  of ‘‘sylvatic plague’’ acknowledges  the ecological significance of Y. pestis among  various  species of wild animals.

There  is some  controversy  about  the  status  of  the  black  rat  in Europe  during  the early Middle  Ages. Adding  to the confusion  is the fact that  ancient  chroniclers  did not distinguish  between rats and mice when  they  spoke  of  ‘‘vermin’’ and  the  strange  behaviors  that  were considered  omens of disaster.  Medieval  physicians and  laymen rightly feared  that  when rats,  mice, moles,  and  other  animals  that  normally lived underground escaped to the surface, acted as if drunk,  and  died in great multitudes,  pestilential disease would follow. These strange por- tents were, however, easily reconciled with the idea that noxious vapors generated deep within the earth could escape into the atmosphere where they produced  deadly miasmata (poisonous  vapors).

Sometime during  the Middle Ages, the black rat  made its way to Europe,  found excellent accommodations in its towns and villages, and took up permanent  residence. The medieval town may seem picturesque through the misty lens of nostalgia, but it was a filthy, unhealthy place of narrow, winding alleys, not unlike rodent warrens, surrounded by haphazard accumulations of garden plots, pig pens, dung heaps, shops, houses, and hovels shared by humans and animals. Perhaps, it is not just a coincidence that a marked decline in the incidence of European plague occurred   at   about   the   same   time   that   the   black   rat   was  being driven out by a newcomer, the large brown rat, Rattus norvegicus.

Although   epidemic  bubonic  plague  may  have  occurred  in  very ancient  periods,  early  descriptions   of  ‘‘plagues  and  pestilences’’ are too  vague to provide  specific diagnoses.  Thus,  the Plague of Justinian in  540 is generally  regarded  as  the  first  plague  epidemic  in  Europe.

Further waves of plague  can  be charted  over  the  next  several centu- ries. Eventually,  the disease seemingly died out in the West, but it was periodically  reintroduced from Mediterranean ports.

According  to  the  historian   Procopius  (ca.  500–562), the  plague began  in Egypt  in 540 and  soon  spread  over  the  entire  earth,  killing men,  women,  and  children  in every nation.  While the  disease always seemed to  spread  inland  from  coastal  regions,  no  human  habitation, no matter  how remote, was spared. Many people saw phantoms before they  were attacked  by the disease, some collapsed  in the streets  as if struck  by  lightning;  others  locked  themselves  into  their  houses  for safety, but phantoms appeared  in their dreams and they too succumbed to  disease. Panic  and  terror  mounted  with  the  death  toll  as civil life ceased; only the corpse-bearers  made their way through  streets littered with rotting  bodies. As the daily toll reached into the thousands, graves and  gravediggers  became  so scarce that  ships were filled with corpses and  abandoned at  sea. Those  who survived  were not  attacked  again, but depravity and licentiousness seemed to consume those who had witnessed and survived the horrors  of the plague.

The sick were the objects of great fear, but  Procopius  noted  that the disease was not necessarily contagious,  because nurses, gravediggers, and even physicians who examined the bodies of the dead and opened plague  buboes  at postmortems might  be spared.  Physicians  could  not predict  which cases would be mild and which would be fatal,  but they came  to  believe that  survival  was  most  common  in  cases where  the plague bubo grew large, ripened, and suppurated. St. Gregory  of Tours (538–594), an  influential  bishop  and  historian,  left an  account  of the plague that  is vague in medical details but vivid in conveying the sense of universal despair. Confused and terrified, the people knew of no appropriate response to the plague other than prayer and flight. Accord- ing  to  Gregory,   large  numbers   of  people  threw  themselves  off  the cliffs into  the  sea ‘‘preferring  a swift death  to  perishing  by lingering torments.’’

There  are  many  gaps  in  our  knowledge  of  the  early  waves  of plague, but  there is no lack of speculation.  Some argue that  the death and  disorder  caused by the plague  led to the decline of the Byzantine Empire. A shift of power in Europe from south to north,  Mediterranean to North  Sea, may have been the consequence of the failure of plague to penetrate  the British Isles, northern Gaul,  and Germania. Establishing the death  toll is virtually  impossible.  Overwhelmed  by panic and  fear, witnesses resorted to symbolic or exaggerated figures to convey the enormity  of the  disaster.  Many  accounts  of medieval  pestilence  state that  mortality  was so great  that  there  were not  enough  of the  living to bury the dead.

Surviving records  are essentially silent about  the status  of plague between the ninth  century  and its catastrophic return  in the fourteenth century. Of course, the absence of specific references to bubonic  plague does not  prove  that  the disease did not  occur  during  that  period.  For the  medieval  chroniclers,  the  causes  of  all great  ‘‘perils and  adversi- ties’’—earthquakes,  floods, famines, pestilential diseases—were beyond human  comprehension or control,  and so common  that  only the most dramatic  were worth recording. During the twelfth and thirteenth centuries, Europe attained  a level of prosperity unknown since the fall of Rome. Population growth began to accelerate  in the eleventh century  and reached  its peak by the four- teenth  century.  Europe  remained  a largely agricultural society, but the growth of towns and cities reflected a demographic and economic revo- lution. Nevertheless, even before the outbreak of plague, conditions were apparently  deteriorating. Famines  had  followed  bad  harvests  in  the years 1257 and 1258. By about 1300, Europe could no longer bring more land into use or significantly improve the yield of land already under cul- tivation.  Wet and chilly weather led to disastrous  harvests from 1315 to 1317. Food  prices soared and malnutrition was more prevalent.

Famines, associated with human and animal sickness, occurred intermittently from  1315 to  1322. Contemporary observers  said  that clergymen  and  nobles  fasted  and  prayed  for  a  pestilence  that  would reduce  the  lower  class population so  that  others  could  live in  more comfort.  If this is true, the fourteenth-century pandemic is an awesome testimonial to the power of prayer. The pandemic that was known as the Black  Death,  the  Great  Pestilence,  or  the  Great  Dying  surpassed  all previous  pestilences  as  a  remedy  for  overpopulation, while  creating more  havoc,  misery,  and  fear  than  any  protagonist on  the  stage  of history before the twentieth  century.

Exactly where or how the Black Death  began is obscure, but many plague outbreaks apparently originated  in the urban  centers of the Near and Middle East. From these foci of infection, plague spread by ship and caravan trade routes. There are many uncertainties about the route taken by the plague and the rapidity of its progress; however, the outline of its journey  by ship  via the  major  ports  of the  Mediterranean and  along the  overland  trade  routes  has  been  charted.  The  ships  of the  Italian city-states  probably  carried  the plague  to western  Europe  in 1347 via the  Crimean  ports  on  the  Black  Sea.  Within  two  years,  the  Great Plague had spread throughout Europe,  reaching even Greenland. Some scholars have argued that the speed with which the Black Death  spread indicates that the great pandemic was not bubonic plague, which usually spreads relatively slowly, but a form of anthrax, typhus, tuberculosis,  or a viral  hemorrhagic fever. Others  have  no  candidates  for  the  disease itself, but insist that  the pandemic was not caused by Y. pestis.

Survivors  of the  plague  years  predicted  that  those  who  had  not experienced  the  great  pestilence  would  never  be  able  to  comprehend the magnitude of the disaster. Indeed, the dispassionate  analytic accounts of historians  attempting to confirm or disconfirm some hypothesis about cause and effect relationships between the plague and subsequent  events make a grim contrast  to eyewitness accounts of the pandemic. Some his- torians see the Black Death as the event that ended the Middle Ages and destroyed medieval social, economic, and political arrangements. Others warn  against  confusing  sequential  relationships  with  cause and  effect. Even the mortality caused by the plague remains a matter of controversy. In some areas, the death rate may have been about 12 percent, whereas in others it exceeded 50 percent. Estimates of the numbers killed in Europe alone range from 20 to 25 million; throughout the world, more than  42 million people may have died of the plague. Repopulation after the Black Death seems to have been quite rapid, but repeated outbreaks of plague, along with other disasters, kept total population levels from rising signifi- cantly until the eighteenth  century.

The plague years provided a significant turning point for the medi-cal profession  and  the clergy. Many  contemporary accounts  speak  of the lack of physicians,  but  it is not  always clear whether  this was due to a high mortality  rate among practitioners or because they had hidden themselves away for fear of contagion.  The effect of the plague on the Church  was undeniably  profound, if also ambiguous.  Mortality among the clergy seems to  have  reached  50 percent  between  1348 and  1349. Mortality in the Pope’s court at Avignon was about 25 percent. In some areas, monasteries,  churches, and whole villages were abandoned. Many writers complained  that  deaths  among  clergymen led to the ordination of men of lower qualifications  and demoralization within the ranks. On the other hand,  fear of death among the general populace  increased the level of bequests to the Church.

With many fourteenth-century physicians convinced  that  a catas-trophic new disease had appeared, hundreds of plague tractates  (treatises devoted to explanations of the disease and suggestions for its prevention and treatment) were written.  Perhaps,  the most compelling account  of the ravages of the plague appears  in Giovanni  Boccaccio’s (1313–1375) introduction to the Decameron, a collection  of stories supposedly  told by ten young men and women who left Florence in an attempt  to escape the plague. According  to Boccaccio, who had survived an attack  of the disease, Florence  become a city of corpses as half of Italy  succumbed to the plague. Very few of the sick recovered,  with or without  medical aid, and most died within three days.

Many died not from the severity of their disease, but from want of care and nursing. The poor were the most pitiable. Unable to escape the city, they died by the thousands and the stench of rotting  corpses over- whelmed  the  city.  Every  morning,  the  streets  were filled with  bodies beyond number. Customary funeral rites were abandoned; corpses were dumped  into  trenches  and  covered with a little dirt.  Famine  followed plague,  because peasants  were too  demoralized  to care for their crops

or their animals. Worse than  the disease itself, Boccacio lamented,  was the barbarous behavior it unleashed. The healthy refused to aid friends, relatives,  or  even their  own  children.  A  few believed that  asceticism would avert the plague, but others took the threat  of death as an excuse for satisfying every base appetite.  Criminal  and immoral  acts could be carried out with impunity  for there was no one left to enforce the laws of man or God.

A surprisingly  cheerful and optimistic  view of the great pestilence was recorded by French  cleric and master of theology, Jean de Venette. According  to de Venette, during the epidemic, no matter  how suddenly men were stricken by the plague, God saw to it that they died ‘‘joyfully’’ after  confessing their sins. Moreover,  the survivors  hastened  to marry and  women commonly  produced  twins and  triplets.  Pope Clement  VI graciously granted absolution to all plague victims who left their worldly goods to the Church.  The Pope sought to win God’s mercy and end the plague with an Easter  pilgrimage to Rome in 1348. The power of faith proved to be no match for the power of pestilence. Prayers, processions, and appeals to all the patron saints were as useless as any medicine pre- scribed by doctors  and quacks.

Guy  de Chauliac,  physician  to  Pope  Clement  VI, confessed that doctors  felt useless and ashamed  because the plague was unresponsive to medical treatment. He noted that the disease appeared  in two forms, one  of  which  caused  buboes  and  another   that   attacked   the  lungs. Physicians,  knowing  the  futility  of  medical  intervention, were afraid to  visit the sick for  fear  of becoming  infected  themselves.  Worse  yet, if they did take care of plague victims they could not expect to collect their fees because patients  almost  always died and escaped their debts. Guy did not join the physicians who fled from Avignon; he contracted the disease, but  recovered.  Pope Clement  VI was more fortunate than his physician.  The Pope remained  shut  up in his innermost  chambers, between two great protective  fires, and refused to see anyone.

Physicians could not cure the plague, but they could offer advice, much of it contradictory, on how to avoid contracting the disease. Aban- doning the affected area was often advised, but opinions varied about the relative  safety  of  potential  retreats.  If  flight  was impossible,  another option  was to turn  one’s home  into  the medieval version of a fall-out shelter.  To reduce  contact  with tainted  air, doctors  suggested  moving about  slowly  while  inhaling  through   aromatic   sponges  or  ‘‘smelling apples’’ containing  exotic and expensive ingredients such as amber and sandalwood, strong  smelling  herbs,  or  garlic,  the  traditional  theriac of the poor.  Bathing  was regarded  as a dangerous  procedure  because baths  opened  the pores and allowed corrupt  air to penetrate  the outer defenses. Physicians eventually developed elaborate protective costumes, featuring long robes, gloves, boots, and ‘‘bird-beaked’’ masks containing a sponge that  had  been steeped  in aromatic  herbs.  In response  to the plague of 1348, many eminent physicians wrote texts called plague regi- mina to express their ideas about  preserving health in dangerous  times. Such texts introduced  readers  to broader  ideas about  health,  including the importance of health  as a public  good,  the importance of policies governing  the sanitary  situation  of towns and  cities, and  the purity  of water, food, and air.

Those fortunate enough  to secure medical attention before being stricken  were  fortified  by  theriac  and  dietary  regimens  designed  to remove  impurities  and  bad  humors.  Once  symptoms  of  the  disease appeared, physicians  prescribed  bleeding  and  purging  and  attempted to hasten  the maturation of buboes  by scarification,  cupping,  cauteri- zation,  poultices,  and  plasters,  which might  contain  pig fat  or pigeon dung.  Some physicians advocated  a direct attack  on the plague  bubo, but a surgeon  or barber-surgeon carried  out the actual  operation. For example, regulations  promulgated by the Health  Board  of Florence  in 1630 directed  the  surgeon  to  apply  cupping  vessels to  the  buboes  or open them with a razor,  dress them with Venice treacle, and cover the surrounding area with pomegranate juice.

During  later  outbreaks of plague,  secular and  clerical authorities attempted to limit the spread of the disaster with prayers and quarantine regulations.  By the fifteenth century, Venice, Florence, and other Italian cities had  developed  detailed  public  health  measures.  Less advanced states throughout Europe used the Italian system as a model for dealing with  epidemic  disease.  Unfortunately, the  well-meaning  officials  who formulated quarantine rules did not  understand the natural history  of plague.  Some measures,  such as the massacre  of dogs and  cats,  must have  been counterproductive. Long  periods  of quarantine—originally a forty-day period of isolation—for those suspected of carrying the contagion  caused unnecessary hardships  and promoted willful dis- obedience.  Modern  authorities  generally consider  a seven-day quaran- tine adequate  evidence that  potential  carriers are not infected.

Antiplague  measures  eventually  included  mandatory reporting  of illness, isolation  of  the  sick,  burning  the  bedding  of  plague  victims, closing schools  and  markets  during  epidemics, virtual  house  arrest  of off-duty   gravediggers,  and  laws  forbidding   physicians  from  leaving infected areas. Plague rules meant extra taxes, destruction of property, restriction  of  commerce,  privation, pest  houses,  and  unemployment. Quarantined  families  were  supposed   to  receive  food  and  medicine, but, as always, poor relief funds were inadequate. Public health officials were supposed  to have absolute  authority in matters  pertaining  to con- trol  of  the  plague,  but  they  often  encountered   noncompliance from members of the clergy. During  epidemics, the secular authorities  could close schools,  prohibit  festivals,  games,  parties,  and  dances,  but  they were generally unable  to stop religious assemblies and processions.

Perhaps, the combination of faith and quarantine, along with more subtle  changes  in  plague  ecology,  eventually  mitigated  the  effects of further  waves of plague, at least in the countryside.  During the fifteenth century,  the rich could expect to escape the plague by fleeing from the city. Eventually, the general pattern of mortality  convinced the elite that plague was a contagious  disease of the poor. However, historical studies of plague  mortality  are  complicated  by diagnostic  confusion  between true bubonic plague and other infectious diseases. In the absence of spe- cific diagnostic  tests,  public  health  authorities  preferred  to  err on the side of  caution  and  were likely to  suspect  plague  given the  slightest provocation. Much ‘‘plague legislation’’ after the Black Death was more concerned  with protecting  the personal  safety and property  of the elite than  with  control  of plague  itself. However,  the  concept  of granting authority to  secular  public  health  officials  was established.  Epidemic plague essentially disappeared  from the western Mediterranean by the eighteenth  century.  Plague remained  a threat  in the eastern  Mediterra- nean  area  well into  the nineteenth  century,  but  later  outbreaks  never achieved the prevalence or virulence of the Black Death.

Plague is still enzootic among wild animals throughout the world, including  Russia,  the  Middle  East,  China,  Southwest  and  Southeast Asia, Africa,  North  and  South  America,  resulting  in sporadic  human cases. Animal reservoirs in the Americas include many different species, but  rats,  mice, marmots,  rabbits,  and  squirrels are the best known.  In Andean  countries,  guinea  pigs raised  indoors  for  food  have  infected humans.  Epidemiologists  studying  emerging  and  re-emerging  diseases warn that unforeseen changes in the ecology of a plague area could trig- ger outbreaks among animals and humans. For example, the movement of  rapidly  expanding  human  populations into  previously  wild  areas raises the risk that plague and other emerging rodent-borne diseases will cause sporadic  cases or even epidemics. Scientists have also speculated about  the possibility that  plague could be used as a biological weapon. However, they generally agree that  only aerosolized  pneumonic  plague could serve as an effective agent.

In the first decade of the twentieth  century,  while California  poli- ticians and merchants acted as if bad publicity was more dangerous  than bubonic plague, the disease escaped from San Francisco  and established itself as an enzootic  disease among  the rodents  of the western  United States.  Plague  was first  officially reported  in San  Francisco  in 1900. When plague bacilli were isolated  from the body  of a Chinese laborer found dead in a Chinatown hotel, the Board of Health  imposed a total quarantine on  Chinatown, in  response  to  both  fear  of  disease  and racism. Even though  22 plague deaths  were officially recorded  in 1900, and additional cases occurred in 1904 and 1907, leading citizens contin- ued to deny the existence of plague. Critics argued that the city and the state  put  business  interests  ahead  of public  health  concerns.  Finally, afraid  that  the city would experience further  outbreaks of plague and, worse  yet,  a  national   boycott,   the  San  Francisco   Citizens’  Health Committee  declared  war  on rats.  Unfortunately, by the time the war had claimed the lives of one million city rats, rodents in the surrounding areas had already  become a new reservoir of plague bacteria.

Prairie  dog  colonies  in  Colorado  provide  a  large  reservoir  of plague,  but  New Mexico has had  the largest  number  of human  cases. The  extent  of  plague  transmission between  rural  and  urban  animals is unknown,  but the danger is not negligible. People have been infected by domestic cats, bobcats,  coyotes, and rabbits.  Because human  plague is  rare  and  unexpected,   sporadic   cases  are  often  misdiagnosed.   If appropriate treatment is not begun soon enough,  the proper  diagnosis may  not  be made  until  the  autopsy.  Almost  20 percent  of the  cases reported   in  the  United   States  between  1949  and  1980  were  fatal. Whereas  only 5 percent  of the cases identified  between 1949 and  1974 were  of  the  pneumonic   form,   between  1975  and  1980  this  highly virulent form accounted  for about  25 percent  of New Mexico’s plague cases. In the United  States, about  10 to 40 cases of plague are reported each  year,  mainly  in  New  Mexico,  Colorado,  Arizona,   California, Oregon,  and  Nevada.

Given the speed of modern  transportation, it is possible for people who have contracted bubonic plague to travel to areas where the disease is unknown  well before  the  end  of  the  two  to  seven-day  incubation period.  One example of the epidemiological  cliche´ that  residents of any city are just a plane  ride away from  diseases peculiar  to any point  on the globe occurred  in 2002 when New York  City health  officials repor- ted two confirmed  cases of bubonic  plague. The victims had apparently contracted the illness in New Mexico, before they left for a vacation  in New York.  They went to  an  emergency room  complaining  of flu-like symptoms,  high fever, headache,  joint pain, and swollen lymph nodes.

The  World  Health  Organization reports  one  thousand to  three thousand cases of plague per year around the world. In some areas, per- haps because of better surveillance or of actual increases in the number of cases, the numbers of suspected and confirmed cases increased during the 1990s. The island nation  of Madagascar, for example, reported  sig- nificant  increases  in the number  of plague  cases. Bubonic  plague  first came to Madagascar via steamboats from India in the 1890s. Although the  disease  had  been  brought   under  control  in  the  1950s,  Y.  pestis remained widely distributed  among the island’s rats and their fleas. Pub- lic health officials discovered that by the 1990s new variants of Y. pestis had emerged, including a multiple antibiotic-resistant strain.

Some historians argue that the pandemic known as the Black Death could not have been caused by Y. pestis, because the fourteenth-century disease  spread  too  quickly  and  was  too  deadly,  and  the  signs  and symptoms were unlike those of modern bubonic plague. Some argue that a bubo or swelling in the lymph glands is not a significant diagnostic sign, because it may occur in filariasis, lymphogranuloma inguinale, glandular fever,  relapsing  fever,  malaria,   typhoid,   typhus,   and  other   tropical diseases. Some historians  contend  that  chronicles of the plague do not mention  major  rat  deaths  and  that  Europe  lacked  rodent  species that could serve as a plague reservoir between outbreaks. There are, however, Arabic sources that describe the deaths of wild and domesticated animals before the epidemic spread  to humans.  In any case, studies of rats and other  pestiferous  rodents  suggest that  it is always wrong to underesti- mate their numbers,  persistence, fertility, and adaptability.

Plague  ‘‘revisionists’’ have  suggested  that  the  Black  Death  was caused by an unknown  microbe that  no longer exists, anthrax, typhus, tuberculosis,   influenza,   a  filovirus,  an  unnamed   viral  hemorrhagic fever, or  Ebola  fever.  Some  historians  have  suggested  that  the  high, but  variable  mortality  rates  reported  for the Great  Dying  might  have been associated with immunosuppression caused by mold toxins. Mycotoxins  could  affect rats  as well as people,  which would  account for  rat  deaths.  Advocates  of  the  ‘‘Ebola  hypothesis’’  argue  that  the most significant  signs of the Black Death  were red spots on the chest, rather  than  the buboes  in the lymph  nodes.  As further  evidence, they argue that  the 40-day quarantine adopted  by public health  authorities corresponds   to  the  latency  and  infectious  period  of  a  hemorrhagic virus.  The  disappearance of  the  disease  in  Europe  during  the  ‘‘little ice age’’ of the late seventeenth and early eighteenth  centuries has been attributed to  a  decrease  in the  infectivity  of the  virus  caused  by the cold,  or  a  mutation in  the  virus.  In  addition  to  putative  changes  in the  hemorrhagic virus,  they  suggest  that  a  possible  genetic  mutation could  have  made  40 to  50 percent  of  Europeans less susceptible  to the hemorrhagic fever virus. Despite uncertainties about  the rapid  dis- semination  of the medieval pandemic  and  the nature  of the European rat  population, the Ebola  hypothesis  itself seems to demand  an exces- sive multiplication of possibilities; it also requires faith in the idea that a lethal tropical  disease achieved global distribution during  the plague years  and   persisted   in  some  unknown   reservoir   even  in  northern regions of the world between outbreaks.

Most epidemiologists believe that Y. pestis was the cause of the dis- ease that  medieval observers called the plague. Because the same agent is transmitted in different ways and can cause different clinical patterns in people,  the  disease caused  by Y. pestis does not  seem inconsistent with historical  accounts  of plague. Many factors have, of course, chan- ged since the fourteenth century, but, even in wealthy nations,  untreated bubonic  plague  has  a  high  mortality  rate  and  victims  of  pneumonic plague have died within 48 hours of exposure to the disease. A compar- ison   of  modern   medical   photographs  with   historical   and   artistic depictions  of plague  victims suggests that  medieval  images of plague victims, saints,  and martyrs  are not  inconsistent  with modern  bubonic plague.  Paintings  of  St.  Roche  typically  show  buboes  in  the  groin. Artists,  however, were often  more  interested  in achieving an aesthetic goal rather  than  a realistic clinical likeness. Medieval  authors  refer to buboes, pustules, or spots, which appeared  on the neck or in the armpit and groin. During  modern  outbreaks, the buboes usually appear  in the groin.  This  seems to  correlate  with  the  fact  that  fleabites  in modern homes  are  generally  no  higher  than  the  ankles.  Of course,  people  in the Middle Ages lived and interacted  with animals and pests far differ- ently, and the pattern of fleabites might have been quite different. Simi- larly,  differences  between  medieval  and  modern  homes,  towns,  and cities suggest that  plague would not spread  in the same manner.

Interesting  evidence of the existence of bubonic plague in medieval Europe  was reported  in 2000 by researchers  who  identified  Y. pestis DNA   in  the  remains  of  bodies  buried  in  France   in  the  fourteenth century.  Critics  who  insist  that  the  Black  Death  was not  caused  by Y. pestis responded  by arguing that  such findings only prove that  some cases of plague occurred  in Europe,  but  the countless  other  victims of the  Black  Death  died  of Ebola  or  some  unknown  disease.  However, the tests that proved positive for Y. pestis did not find evidence of other possible  causes  of  the  Black  Death  in  bodies  from  the  mass  grave. Attempts  to exonerate  rats,  fleas, and Y. pestis have encouraged  more sophisticated analyses of the history of plague but have not been com- pelling in their support  for alternative  hypotheses.

Even  after  the  publication of  the  genome  of  Y.  pestis  and  the warning  that  such  information would  be  of  interest  to  bioterrorists, perhaps  the most  dangerous  characteristic  of bubonic  plague  today  is its ability to camouflage itself as a ‘‘medieval plague’’ of no possible sig- nificance to modern  societies. Much about  the disappearance of plague from the ranks of the major epidemic diseases is obscure, but we can say with a fair degree of certainty  that  medical breakthroughs had little to do with it. In its animal  reservoirs,  the plague  is very much  alive and presumably   quite  capable  of  taking  advantage   of  any  disaster  that would  significantly  alter  the  ecological  relationships among  rodents, fleas, microbes, and human  beings.

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