There are three distinct clinical forms of plague caused by the bacterium Yersinia pestis: bubonic (affecting the lymph nodes), pneumonic (lungs), and systemic (entire body). In the bubonic form, which is the most common, the lymph nodes become painfully enlarged (particularly in the groin area), forming buboes. Symptoms include high fever, rapid and irregular pulse, hemorrhage, prostration, delirium, shock, coma, and death within 3 to 5 days. Another name for this disease, the Black Death, reflects the fact that plague hemorrhages often take on a blackish discoloration.

Although pneumonic plague is mainly spread through the air by droplets, bubonic plague transmission involves the rat flea as a vector. The disease is actually a zoonosis, with rats the primary host. Once enough rats die, however, the infected fleas look for other hosts, including humans.

Epidemics of this disease repeatedly killed more than a fourth of the population of Europe during the Middle Ages and thus is partly responsible for this period being referred to as the Dark Ages there. Only malaria and tuberculosis have been more deadly. The original onset of the Black Death midway through the fourteenth century has been linked to the Crusaders, whose return across the Mediterranean brought the black rat (Rattus rattus) as a hidden stowaway on their ships. The resulting first pandemic spread of plague throughout Europe and parts of Asia is believed to have killed as much as three-fourths of the population in a period of less than 20 years. The disease is still prevalent in some areas of the world (an outbreak occurred in India in 1994), but antibiotics have greatly reduced the mortality rate. The few cases that occur in the United States each year are mainly in the southwest, where the disease appears to be endemic (but usually not fatal) among ground squirrels and other wild rodents.

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