Factors affecting the spread and maintenance of plague.

Plague is an exceptionally virulent fl ea-borne illness caused by the gram-negative bacterium Yersinia pestis (Prentice and Rahalison 2007 ). Humans are accidental hosts of this bacterium, which normally circulates among certain rodent species and their fl eas, occasionally causing widespread plague epizootics with high mortality among its hosts. Most people have little knowledge of plague’s status in the modern world, although many are aware that the disease causes outbreaks with high mortality and can spread very quickly within human populations. Some also remember that plague was the cause of the Black Death, an explosive epidemic that killed perhaps one-third of Europe’s population over an approximately 4 years period in the mid-fourteenth century (Carniel 2008 ) . Although the Black Death is the most widely recognized pandemic, plague also caused two other less well-known pandemics (Justinian’s Plague and the Modern Pandemic) that killed millions, as well as innumerable regional epidemics, some of which caused the deaths of tens of thousands. In addition to causing high mortality, these outbreaks were characterized by the explosive spread of plague among its victims. Humans typically acquire plague through one of three routes of exposures: infectious fl ea bites, handling infected animals or inhaling infectious materials. The rarest source of exposure is inhalation of infectious materials, which typically results in primary pneumonic plague, a form of the disease that is characterized by very rapid onset, fever, and respiratory symptoms, including cough and frequently hemoptysis (expectoration of bloody sputum carrying viable Y. pestis ). In most instances, primary pneumonic plague is acquired following exposure to another person who has pneumonic plague with associated cough and hemoptysis. Untreated cases of pneumonic plague are especially dangerous because they can lead to very rapid person to person spread and serious epidemics with exceptionally high mortality rates in the absence of appropriate control measures. Such outbreaks appear to have occurred regionally during the Black Death and others were con fi rmed early in the twentieth century in Manchuria during the Modern Pandemic, causing as many as 50,000–60,000 deaths (Meyer 1961 ; Pollitzer 1954 ) . Factors promoting the spread of pneumonic plague include crowding of people within con fi ned spaces, family members or others providing care to coughing patients without the use of appropriate respiratory protection, and cool and relatively humid conditions (Gabastou et al. 2000 ; Meyer 1961 ) . In other instances humans are exposed to plague through direct contact with the infectious body fl uids and tissues of infected animals. Cases Factors Affecting the Spread and Maintenance of Plague