Bubonic Plague

Bubonic plague is an infectious disease that is believed to have caused several epidemics or pandemics throughout history.

Infection
The disease is caused by the bacterium Yersinia pestis and is usually transmitted by the bite of fleas from an infected host, often a rat. The bacteria are transferred from the blood of infected rats to the Rat Flea (Xenopsylla cheopsis). The bacillus multiplies in the stomach of the flea, blocking it. When the flea next bites a mammal, the consumed blood is regurgitated along with the bacillus into the bloodstream of the bitten animal. Any serious outbreak of plague is started by other disease outbreaks in the rodent population. During these outbreaks, infected fleas that have lost their normal hosts seek other sources of blood.

Symptoms and treatment
The disease becomes evident 2-6 days after infection. Initial symptoms are chills, fever, headaches, and the formation of buboes. The buboes are formed by the infection of the lymph nodes, which swell and become prominent. If unchecked, the bacteria infect the bloodstream (septicemic plague) and then the lungs (pneumonic plague).

In septicemic plague there is bleeding into the skin and other organs, which creates black patches on the skin, hence the name Black Death. Mortality in untreated cases is 50-90%, but early treatment with antibiotics is effective (usually streptomycin or gentamycin), reducing the mortality rate to around 15% (USA 1980s).

With pneumonic plague the infected lungs raised the possibility of person-to-person transmission through respiratory droplets. After two to four days of incubation the initial symptoms of headache, weakness, and coughing with hemoptysis are indistinguishable from other respiratory illnesses. Without diagnosis and treatment the infection can be fatal in one to six days, mortality in untreated cases may be as high as 95%. The disease can be effectively treated with antibiotics, however.

As a biological weapon aerosolized pneumonic plague is the only effective plague agent.

Historic outbreaks
A special warning has to be made about early epidemics of the "plague", for example in Greek or Roman history or in the Bible - these are usually not well enough documented to make any definite statement about the nature of the disease; the usage of the name stems from the early modern time, when the plague was the only disease known to cause massively killing epidemics.

Many scientists believe that there was an outbreak of bubonic plague in the 6th century, starting in Africa and moving to Constantinople and the rest of the Byzantine Empire.

Most scientists believe that the Black Death in the 14th century was an outbreak of bubonic plague. However, other theories have now been advanced, suggesting that the Black Death may have been an outbreak of some other disease, possibly a hemorrhagic fever similar to Ebola, or anthrax.

The Great Plague of 1665 in London is also generally believed to have been an outbreak of bubonic plague.

After a localised outbreak in Provence in southern France in 1720-1721, Europe suffered no more such attacks of plague, though the disease remained virulent in other regions, killing upwards of ten million in India in the late nineteenth and early twentieth centuries according to some estimates.

The last rat-borne epidemic in the United States occurred in Los Angeles, California in 1924-1925.

Contemporary cases
The disease still exists in wild animal populations in the Caucasus Mountains in Russia, through much of the Middle East, China, Southwest and Southeast Asia, Southern and Eastern Africa, in North America from the Pacific Coast eastward to the western Great Plains and from British Columbia southward to Mexico, and in South America in two areas - the Andes mountains and Brazil. There is no plague-infected animal population in Europe or Australia.

Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year.

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