Bubonic plague, the most common form of the disease, is acquired directly from the bite of an infected flea. Other, less common forms of plague include pneumonic, which can develop from the bubonic form and is spread directly from person to person by the respiratory route; septicemic; and meningeal, or plague meningitis. Bubonic plague derives its name from the characteristic swollen lymph nodes (called buboes) in the groin, axilla, and neck areas. Untreated bubonic plague is fatal in half of all cases; untreated pneumonic plague is invariably fatal.
Plague is probably best known because of its role in the Black Death (so called because of the gangrenous extremities often seen in those with advanced disease), a devastating pandemic that swept through much of Asia and Europe during the Middle Ages. Some 20 million people, representing 20 to 25 percent of Western Europe's total populace, are thought to have died during a four-year period (McEvedy, 1988). In large European cities during the peak of the epidemic, people died in such great numbers that few were left to bury the dead. An accurate count of plague victims—especially the poor, who lived in the most crowded conditions—was impossible.
The Black Death arrived in Europe from central Asia in 1346, probably by the ''Silk Road," a trading route from Asia to Europe. It reached Italy by ship from Caffa (McEvedy, 1988). Like many pandemics, its spread was made possible by the devices of transportation. Medical historians believe that the plague bacillus was endemic in the marmot population of Central Asia. It is likely that Mongol invaders who killed marmots for their fur inadvertently extended the range of the Y. pestis-infected rodents and their fleas. The Mongols often rode long distances in a single day and may thereby have transported an occasional infected rat or some fleas as they moved along the trade routes toward Europe (McNeill, 1976).
In the late 1890s, bubonic plague appears to have been introduced into San Francisco by infected rats traveling aboard an Asian merchant ship. In 1900, a small outbreak of the disease developed in the Chinese population of San Francisco. That same year, ground squirrels were found to be infected; soon the plague bacillus had infected most of the area's burrowing rodents (McNeill, 1976). Plauge infection is now enzootic in much of the rodent population in the western United States, Mexico, and Canada.
Thanks to modern sanitation and the availability of antibiotics and pesticides, another occurrence of the Black Death seems unlikely. Still, an outbreak is not out of the question, particularly in regions, including the western United States and parts of South America, Africa, and Asia, where wild rodent populations are persistently infected with the plague bacillus. Isolated cases of plague are reported to this day in various parts of the world, including the United States. Crowding and poor sanitation, if they occur in or near areas where rodent plague is enzootic, could provide conditions for the reemergence of this once devastating bacterial illness. Current legal