Bubonic plague natural history

Plague has a remarkable place in history. For centuries, plague represented disaster for those living in Asia, Africa and Europe, where, it has been   said, populations were so affected that sometimes there were not enough people left alive to bury the dead. Because the cause of plague was unknown, plague outbreaks contributed to massive panic in cities and countries where it appeared.

Perspective and history
Plague has a remarkable place in history. For centuries, plague represented        disaster for those living in Asia, Africa and Europe, where, it has been         said, populations were so affected that sometimes there were not enough         people left alive to bury the dead (Gross, 1995). Because the cause of        plague was unknown, plague outbreaks contributed to massive panic in cities         and countries where it appeared. The disease was believed to be delivered        upon the people by the displeasure of the gods, by other supernatural         powers or, by heavenly disturbance. Innocent groups of people were blamed        for spreading plague and were persecuted by the panicked masses. Numerous        references in art, literature and monuments attest to the horrors and         devastation of past plague epidemics. So imprinted in our minds is the        fear of plague that, even now, entering into the 21st century, a suspected         plague outbreak can incite mass panic and bring much of the world's economy         to a temporary standstill. The number of human plague infections is low        when compared to diseases caused by other agents, yet plague invokes an         intense, irrational fear, disproportionate to its transmission potential         in the post-antibiotic/vaccination era.

Fundamental works
The fundamental but separate works by Yersin and Kitasato in        1894 on the discovery of the etiologic agent of plague in Hong Kong opened         the way for investigating the disease and how it is spread. Kitasato and        Yersin described, within days of each other’s findings, the presence         of bipolar staining organisms in the swollen lymph node (bubo), blood,         lungs, liver and spleen of dead patients (Bibel et al., 1976). Cultures        isolated from patient specimens were inoculated into a variety of laboratory         animals, including mice. These animals died within days after injection,        and the same bacilli as those found in patient specimens were present         in the animal organs. Though both investigators reported their findings,        there were a series of confusing and contradictory statements by Kitasato         that eventually led to the acceptance of Yersin as the primary discoverer         of the organism now named after him, Yersinia         pestis (Bibel et al., 1976). Yersin had recorded that rats were        affected by plague not only during plague epidemics but also often preceding         such epidemics in humans. In fact, plague was designated, in local languages,        as a disease of the rats: villagers in China, India and Formosa (Taiwan)         described that when hundreds and thousands of rats lie dead in and out         of houses, plague outbreaks in people soon followed (Gross, 1995). The        transmission of plague was described by Simond in 1898. He noted that        persons who became ill did not have to be in close contact with each other         to acquire the disease. In Yunnan, China, inhabitants would run away from        their homes as soon as they saw dead rats. On the island of Formosa, residents        considered handling dead rats a risk for developing plague. These observations        led Simond to suspect that the flea might be an intermediary factor in         the transmission of plague since people acquired plague only if they were         in contact with recently dead rats and were not affected if they touched         rats that were dead for more than 24 hours. Simond demonstrated that the        rat flea (Xenopsylla cheopis) transmitted the disease in a now         classic experiment in which a healthy rat, separated from direct contact         with a recently plague-killed rat, died of plague after the infected fleas         jumped from the first rat to the second.

Ancient disease
Plague is an ancient disease that is not likely to disappear;        its continued outbreaks throughout the world attest to its tenacious presence. Since the first descriptions, many studies have examined the transmission,        epidemiology and pathogenesis of the disease (Gage, 1998). Plague is a        bacterial infection of small mammals transmitted from animal to animal         by the bite of infected fleas. Plague cycles naturally in its enzootic        foci, circulating between small mammals and fleas without human involvement. The quiescent periods, during which few or no human cases are detected,        may last for years, leading to mistaken declarations of plague eradication. However long the silent periods last, plague may suddenly reappear. The        combination of false assurance of its eradication, and the failure of         public health vigilance, sets the stage for the panic that may ensue when         enzootic plague spills over from its natural cycle into the peridomestic         and commensal rodent populations (and their fleas), bringing plague into         closer human contact. Poor sanitation, overcrowding and high numbers of        rodents are conditions that enhance urban plague transmission. Thus, a        plague outbreak has come to represent an indictment of social, environmental         and political changes in the modern world.