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The Black Plague of the mid-1300's is the most famous occurrence of Bubonic Plague, which repeatedly ravished the Eurasian population prior to the Industrial Age. Bubonic Plague is caused by Yersinia Pestis, a bacteria that lives in the gut of a common rat flea, Xenopsylla cheopis. When a flea bites an infected rat, it consumes the bacteria, which multiplies and reproduces inside the flea's intestinal tract, until the flea has no room to contain them anymore and regurgitates them onto its sucker. From the leftover bacteria on the sucker, the next rat bitten by the flea gets a healthy dose of Yersinia Pestis and the cycle starts all over again.
Although the flea that carries plague prefers to feed on rats, live rats are not always available to them. Yersinia Pestis produces a toxin (Necrotizing Exotoxin) that causes rats and humans to suffer similar symptoms, including swollen lymph nodes, hemorrhages in the lower layers of skin and finally death. A population of rats living nearby each other will all catch Plague and die from it, leaving their fleas to seek other animal hosts, including humans and many domestic animals.
For humans, sickness from plague comes in two forms: bubonic and pneumonic. All of the infections are caused by the same bacteria and start in the same way, mainly when the bacteria enter the human body. The Black Plague of the 14th century is the best known example of Bubonic Plague. The toxin produced by Yersinia Pestis caused people's lymph nodes to swell up into "buboes" which looked like huge, hard bubbles sticking out of a person's skin. Although the era is famous for its buboes, at the same time, plague was spreading pneumatically and septemically.
In some of the people infected by fleas and their neighbors, the bacteria moved into their lungs rather than their lymph nodes. Bacteria and fluids in the lungs caused hemorrhages, so that people essentially drowned a mixture of their own blood and toxins from the bacteria. Before they died, they coughed liquids up, and these contaminated the water supply, food and air of anyone who came to see them, or cared for them. Prior to the advent of antibiotics, pneumonic plague was the most deadly of the three, with a mortality rate of nearly 100% in the first 48 hours.
Though the Black Plague is one of the best documented epidemics of the disease ever, it is not the first recorded incidence of an outbreak. Yersinia Pestis has been with humanity for a long time. The first documented case of what came to be called plague was in 1320 BC, shortly after the Philistines removed the Ark of the Covenant from the Israelite temple. Plague returned in Egypt in 542 AD, during the reign of the Roman emperor Justinian. This epidemic lasted 60 years and may have killed as many as 100 million people in Central Asia, Northeast Africa and the Roman empire.
Although isolated incidents of plague likely occurred between epidemics, the next time plague came for a long visit at the human table was in the early 1330's when a pandemic began in China, where it spread over land to Mesopotamia and Asia Minor. Genoese traders brought the disease back to Europe in 1348 and by 1351 it had spread to Russia and northern Europe, where it turned into the Black Plague. In Europe it lasted between five and ten years, but was followed by a second wave in the early 1360's. Plague seemed to go dormant in the winter when the rat flea Xenopsylla cheopis, went dormant. At the end of the outbreak, approximately one-quarter to one-third of the population of Europe had died. Massive social changes ensued as labor became a saleable commodity, giving people new latitude to find "competitive wages". Faith in the Roman Catholic Church was shaken by the Church's inability to do anything for the ill. Some sources consider the Black Plague to be a causative agent of the Renaissance -- after so much death, Europe needed a rebirth.
Though the pandemic ended, plague didn't leave the human population. In 1665, 70,000 died in London as a result of plague and more recently, in 1885 an epidemic broke out in Asia, killing 10 million. Nine years later, plague broke again in China's Yunan province.
During this epidemic, the bacteria Yersinia Pestis was first isolated by scientists Alexandre Yersin and Shibasaburo Kitsato. Once these scientists were able to bring samples back to the lab, work was begun on researching what exactly plague was and how it could be combated. Several decades later, it became apparent that modern antibiotics were effective in treating plague.
However, treatment of plague is nothing like treatment of strep throat or ear infections, which are also caused by bacteria. Only one family of antibiotics, called tetracyclines, work on Yersinia Pestis. Sometimes they are administered in combination with streptomycin, but tetracyclines must always be present in the antibiotic mixture or plague is unaffected. In case of bubonic plague, the antibiotics must be administered within 24 hours of exposure, or they are rendered useless. Pneumonic form is even more virile; if treatment is not begun within 15 hours, survival rate returns to almost zero, where it was during the Black Plague.
Unfortunately, Yersinia Pestis and rat fleas did not die out with the Industrial Revolution. Yersinia Pestis is a permanent resident in the fleas of rats and other small mammals in several regions of the world, including ground squirrels in the Southwestern United States and Manchurian Marmots in China. Several cases of plague have appeared in the past decade in New Mexico and other Southwestern States, as well as China and Madagascar. Although any case of plague is a concern, the strain of Yersinia Pestis in Madagascar is particularly troubling because it seems to have complete resistance to antibiotics. Though medical professionals are quickly working on new, slightly altered tetracycline derivatives, there's no saying they will be successful before one of these strains becomes a major problem, at least in Madagascar, and possibly worldwide.
Preventative measures and sanity conditions are the next best thing when antibiotics fail. Water and sewage treatment facilities and sterile hospital conditions contribute enormously to preventing plague and other infectious diseases from turning into national epidemics in the United States. Cleaning up dirty city streets and rat infestations, leaving sickly stray and wild animals to the care of professional shelters and pest removal services, and promptly taking anyone to the hospital who manifests symptoms of the bubonic or pneumonic infection are also crucial to keep incidents isolated.
There's little reason to fear infection by plague in the US, except in specific towns or cities in the Southwest where recent cases have been reported. If planning to go anywhere that might infested with rats either in the US, or particularly in a third world nation, check local current events for any cases of bubonic or pneumonic plague or infections of Yersinia Pestis.