In 6000 B.C. there were fewer humans on earth than now occupy New York City and Tokyo. Those prehistoric residents were scattered over vast expanses of the planet and few ever ventured far from their birthplace. Their primary microbial threats were from parasites or occasional encounters with zoonotic disease.

After the last ice age 10,000 years ago, and with the benefit of domestication, human populations grew and congregated around rivers, ports, and food resources. Trade routes emerged connecting the urban centers. By 3000 BC, cities and states emerged. With their achievements in science, art, and warfare came the conditions for epidemic diseases.

One of the most fascinating is Plague, a disease that has changed history. Do understand this at the outset: plague is a disease. The Black Death is the historical event when the devastating disease swept over Europe.

First, plague the disease. The causative organism is called either Pasturella pestis or Yersinia pestis. Plague is primarily a disease of rats and wild rodents and is transmitted from animal to animal by the bites of infected fleas, with humans as the accidental host. Vaccines have existed since 1898. Streptomycin or tetracycline are effective antibiotics. By definition, plague is a - zoonotic disease: a disease that can be transmitted to people from animals in the wild.

Many different kinds of rodents are infected with the plague bacillus, among them marmots, ground squirrels, prairie dogs, chipmunks, gerbils, rats, mice, and rabbits. Sometimes the disease is spread from field rodents to species of rats that live in close association with humans. The bacteria are spread from rodent to rodent by fleas. When a flea feeds on an infected animal, its gizzard becomes 'blocked' with masses of plague bacilli so that it cannot suck blood effectively. In trying unsuccessfully to feed again, it regurgitates blood and bacilli into another animal.

Various species of fleas can transmit the plague bacillus, but one of the most effective is Xenopsylla cheopis, a flea of an indoor rat, the black rat. This species of flea is more likely to venture out and attack humans than some other flea species, which prefer to stay in the rat's nest. When a rodent dies of the plague, its fleas leave the cold body and look for another host. Now a human may accidentally be bitten and infected.

When the plague bacillus in enzooic or epidemic in wild rats, is called silvatic plague (a plague epidemic amongst animals). This provides a reservoir which can last for long periods.

When a human is infected by flea bites, it is called bubonic plague. The organisms spreads to the lymph nodes, which swell and are called buboes. From there, the organism can spread to the blood stream causing septicemic plague. This disease kills 50-60% of its victims in the bubonic form, and is universally fatal in the septicemia form.

In the pneumonic plague, the disease becomes a lung infection and the disease is spread by coughing. Spread is rapid and is devastating. It was easily spread inside the densely populated medieval cities of Europe. Why the name, black plague? The patient dies cyanotic--a black color. Once the rat driven bubonic form was established in crowded urban centers, the pneumonic form soon appeared, with deadly consequences.



Plague is a fascinating disease of history. It survives today, ready to break out again. It came to the western United States a century ago and became established in the wild rodent populations.. It infects animals from New Mexico prairie dogs to California squirrels. More than 300 individuals have contracted plague in the United States since 1947, mostly in the West. It often gets to people by way of domesticated house cats.

According to the CDC's national plague center in Fort Collins, Colorado, this is one of the three remaining diseases for which people can be quarantined--along with cholera and yellow fever.

An outbreak of plague in India in 1994 drew world attention. Poverty, war and jet travel help these disease organisms travel and find new victims. During that outbreak in India in 1994, plane crews on flights originating in India were required to notify U.S. health officials if a passenger on the plane was sick. If so, the plane would be met by a U.S. health official out on the tarmac and quarantined on the spot.



Elsewhere we discuss ergotism, its health and social implications. There is a connection with historical outbreaks of plague. During wet years, mycotoxins caused immunosuppression in humans and death in rats. Fleas would leave the rats and seek out hosts--often humans. People were bitten, infected, and the disease quickly took hold in persons with impaired immune systems due to ergotism. Historically, plague was less terrible in places that were cooler and drier.



An epidemic of plague is colorfully described in the Bible, Samuel 5:6-12 with mention of tumors (buboes), swarms of rats, death and destruction. Plague is also mentioned in the annals of the Assyrian king, Sennacherib (705-681 BC). There was plague in ancient Greece, where Apollo was said to have sent mice carrying plague. By the 6th century it reached Arabia.

By 542, Egypt, Palestine, Syria and Europe had experienced bubonic plague. It infected the Roman Empire during the reign of Justinian and contributed directly to the collapse of the Empire. The plagues ended when victims died and the rat population was no longer large enough to carry the disease in epidemic proportions.

The world has experienced at least two great pandemics of bubonic/pneumonic plague. Although the bacterium Y. pestis remains endemic in animal populations, ideal ecological conditions for its rapid spread amongst people has only occurred a handful of times in recorded human history.



Sometime around 1346, the Black Death began on the steppes of Mongolia: infected fleas infested millions of rodents which, in turn, raided human dwellings in search of food. Why the disease emerged that year is not clear, but prolonged wet weather may have been a crucial factor. That weather favored a rodent population explosion. The disease spread rapidly along trade routes. Rumors of the Asian plague told of India, China, and Asia Minor covered with dead bodies. The Chinese population plunged from 123 million to 65 million due to the plague and famine that followed.

Plague reached Sicily in 1347. Ailing men passed the disease directly and rats carrying fleas spread into the local rodent population. As the plague made its way across Europe and North Africa, each city anticipated its arrival and tried by a variety of means to protect itself. Travelers were barred entry, drawbridges were raised to seal the wealthy urbanites off from the less fortunate peasantry. Jews were blamed--and killed.

Some who had no scapegoats blamed the plague on their own lack of piety. The Brotherhood of Flagellants were Christian men who daily beat themselves to the edge of death to purge the sins that were responsible for their disease. All over Europe, these men, encouraged by crowds of crazed aristocrats and peasants alike, would thrash themselves with leather whips embedded with small iron spikes.

Each city would be in the grip of the disease for four or five months until the susceptible rats and humans had died. The survivors would then face famine and economic collapse, caused by the sharp reduction in the workforce. One-third of Europe's total population died of plague between 1346 and 1350.

Why was Europe so vulnerable? In large cities, people lived in filth. Bathing was considered unhealthy. In one small room, a dozen people slept on the floor. Dead animals littered the streets and beggars displayed sores and deformities for alms. Excrement flowed down walls, often into rivers. Animals were slaughtered in public and remains left to rot. Rodents and vermin were the constant companions of people.

The Black Death in Europe and England utterly changed society. By 1351, the Black Death had run its course. Some census records put the death toll at about 25,000,000 persons. The more populous a city, the higher the mortality rate. Walled cities were extremely deadly.

The major effect of the Black Death was to loosen the economic, social, and religious bonds that had been the glue of pre-plague Europe. There was economic chaos, social unrest, profiteering, moral disintegration characterized by wild expenditures, debauchery, social and religious hysteria, greed, maladministration and a decay of manners.

There was a loss of manpower due to the high mortality. The severe reduction in the labor force drove up wages and made farm workers more mobile, accelerating the disintegration of the manorial system. The Church lost prestige and authority as a result of the plague; though religious fervor increased, it found expression through dissident sects. The tragic pessimism of the times pervaded the arts as well--death became a dominant theme in paintings.

Crews of ships were left unmanned. Entire towns were left devoid of people. In southern France, mortality was so great that the Pope consecrated the river Rhone at Avignon so corpses tossed in the river might be considered as having received a Christian burial. Afflicted settlements in Greenland died out. Buildings began to fall apart for lack of workers to repair them.

Economically, the Black Death produced revolutionary changes. Previous bonds of servitude were broken. There was such a shortage of workers that after a plague vanished, the time afterward was a 'golden age of labor.

In England, the labor shortage encouraged a change from crop farming to sheep herding--a subsistence requiring fewer workers. The Protestant Reformation, the emigration of the Pilgrims, and the founding of Pennsylvania by the Quaker William Penn were in no small measure a result of the Black Death.

For two centuries after the historical Black Death in the 1300s, severe plague occurred every several years. In the Muslim empire of the Middle East, the Black Death and the recurring epidemics of the next two centuries led to population decline and disruption of the economy. Several years of extremely high or low water levels on the Nile River combined with the shortage of agricultural workers to produce famine. Muslim beliefs prescribed rather different responses to the plague than did the Christian religion in Europe. While Christians interpreted the plague as God's punishment for their sins, Muslims tended to view it as a calamity decreed by an unknowable God. Death was not a punishment but, if anything, a martyrdom. Because the plague was fated by God, flight from it was discouraged, although some people did flee as the disease approached.

Plague became a weapon of war in an unusual way. A Tartar officer, while besieging a trading post on the Black Sea, began loading the bodies of his men who had died onto catapults and heaved them over city walls. Fleas fled from the dead bodies and were the instruments in a deadly biological warfare.

A great plague pandemic occurred just a century ago and came to this country. In the 1890s, over 13 million people died within forty years. Most of these deaths were in India, but the disease spread widely from one seaport to the next. Plague entered the port of San Francisco and spread to the rat infested warehouses. Eighty-nine people died in San Francisco in the 1907-1908 epidemic, but perhaps more significant in the long run is that plague became established for the first time in the wild rodent population in the United States.

Why did plague disappear? Since isolation of the causative organism and development of a successful vaccine made from killed organisms a hundred years ago, prevention and treatment are now reasonably successful. Pneumonic plague, however, still is so dangerous that attendants of the sick must wear masks, gowns, and gloves. But the problem remains, because plague has faded in times past as remarkably as it appeared. One theory involves rats. The black rat which lived amiable with people in the past has been displaced more recently by the more vigorous brown rat. It is less compatible with people--and people are less likely to be infected by their fleas.

With rural reservoirs of plague in so many parts of the world, epidemics spreading into populations of city rats are always a possibility. Outbreaks in India in 1994 resulted in people fleeing to other parts of the country. In some areas of India, rats are revered and are even fed by people.

Rat resistance to rat poisons and flea resistance to insecticides are problems today. Plague broke out in Viet Nam under wartime conditions in the late 1960s. Wherever social conditions deteriorate even today, people may again get plague from rats.

..... CJ '97


Cartwright, F. Disease and History. New York: Dorset Press, 1972.

Garett, L. The Coming Plague. New York:Farrar, Straus and Giroux, 1994.

"Pneumonic Plague" The Wall Street Journal Oct, 1994.

Smith, D. and Connant, N. Zinsser Microbiolgy. Appleton Century Crofts, 1960.

Recognition: This is a special 'thank you' to John Peck for his informative term paper in 1994.