The Disease In Humans

During the last 2000 years, three great plague pandemics, including the Black Death of the 14 th century, have resulted in social and economic upheavals that are unmatched by armed conflicts or any other infectious disease. Plague, caused by the rod-shaped, gram-negative bacterium Yersinia pestis (Pasteurella pestis was the name used before 1970), is a zoonotic infection, transmitted by any one of several species of fleas, that predominantly affects small mammals such as rodents; humans actually are accidental hosts.

At present, most human cases of the plague are of the bubonic form, which results from the bite of a flea, usually the common rat flea Xenopsylla cheopis, that has fed on an infected rodent. The bacteria spread to the lymph nodes (armpits and neck but frequently the area of the groin) that drain the site of the bite, and these swollen and tender lymph nodes give the classic sign of bubonic plague, the bubo (from the Greek word boubon meaning groin). Three days after the buboes appear, there is a high fever, the infected individual becomes delirious, and hemorrhages in the skin result in black splotches. Some contend that these dark spots on the skin gave the disease the name Black Death, whereas others believe "black" is simply a mistranslation of "pestis atra" meaning not black, but a dark or sinister disease.

The buboes continue to enlarge, sometimes reaching the size of a hen's egg, and when these buboes burst there is agonizing pain. Death can come 2 to 4 days after the onset of symptoms. Sometimes, however, the bacteria enter the bloodstream. This second form of the disease, which may occur without the development of buboes, is called septicemic plague. Septicemic plague is characterized by fever, chills, headache, malaise, massive hemorrhaging, and death. Septicemic plague has a higher mortality than bubonic plague.

In addition, the bacteria may move via the bloodstream to the alveolar spaces of the lungs, leading to a suppurating pneumonia or pneumonic plague. Pneumonic plague, the only form of the disease that allows for human-to-human transmission, is characterized by a watery and sometimes bloody sputum containing live bacteria. Coughing and spitting produce airborne droplets laden with the highly infectious bacteria, and by inhalation others may become infected. Pneumonic plague is the rapidly fatal form of the disease, and death can occur within 24 h of exposure. It is likely that this form of transmission produced the devastating Black Death. The nursery rhyme "Ring around the rosies, a pocket full of posies, Achoo! Achoo! We all fall down" refers to plague in 17th-century England: the rosies are the initial pink body rash, posies the perfumed bunches of flowers used to ward off the stench of death, "achoo" is the coughing and sneezing, and death is signified by "we all fall down."

Y. pestis is one of the most pathogenic bacteria: the lethal dose that will kill 50% of exposed mice is only a single bacterium that is injected intravenously. Typically, Y. pestis is spread from rodent to rodent by flea bites, but it can also survive for a few days in a decaying corpse and can persist for years in a frozen body.

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