Bubonic Plague

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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 manipulate 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 laboratory 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 devastating 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 clinical 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

Bubonic Plague Resistance
Religious depiction of proper responses to the plague.

symptoms include restlessness, anxiety, mental confusion, hallucinations, and coma. Certain bacterial proteins inhibit the immune responses that would otherwise block the multiplication and dissemination 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 throughout the bloodstream, damaging internal organs and blood vessels, leading 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, pneumonic plague is highly contagious and exceptionally lethal. Just what circumstances 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 containing hordes of bacteria. When inhaled into the respiratory system of new hosts, plague bacteria multiply rapidly, resulting in the highly contagious 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. pseudotuberculosis genes, and establishing a remarkable pattern of chromosomal 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 biological 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 epidemiologists 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 twentieth 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, Emil 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 communities 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 disease 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 contrast 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 ofthe bacillus, bubonic plague could again emerge as a very serious threat.

Although Y. pestis can easily penetrate mucous membranes, it cannot 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 primary 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 conditions 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 portents 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 centuries. 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. According 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 adversities''—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 fourteenth century. Europe remained a largely agricultural society, but the growth of towns and cities reflected a demographic and economic revolution. 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 cultivation. 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 historians 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 significantly until the eighteenth century.

The plague years provided a significant turning point for the medical 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 catastrophic 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 overwhelmed 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 prescribed 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. Abandoning 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, cauterization, 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 disobedience. Modern authorities generally consider a seven-day quarantine 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 control 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 specific 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 Mediterranean 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 trigger 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 politicians 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 continued 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 reported 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, perhaps 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 significant 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. Public 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 underestimate 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 dissemination of the medieval pandemic and the nature of the European rat population, the Ebola hypothesis itself seems to demand an excessive 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 disease 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, changed 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 comparison 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 differently, and the pattern of fleabites might have been quite different. Similarly, 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 compelling 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 significance 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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