The plague is arguably the most infamous and feared disease in human history. It is caused by the bacteria Yersinia pestis, which is spread primarily by rats and their fleas. When a flea is infected with the plague, the bacteria blocks its stomach and makes feeding difficult; the flea then becomes more aggressive as it feeds, regurgitating the bacteria into the bite wounds, and transmitting the infection to the host. When infected rat fleas bite humans, they infect them with bubonic plague, which causes swelling in the lymph nodes around the bite. Yersinia pestis simultaneously kills infected cells while preventing their communication with the body’s immune system, which means that the disease is almost always fatal if left untreated. The bubonic plague is the most common form of the disease. However, the plague can also become pneumonic if the infection enters the lungs, or septicemic if it enters the blood stream; both of which are rarer, but more fatal than the bubonic plague. The three major plague pandemics have been traced back to Central Asia, where colonies of rodents act as natural reservoirs in which the disease has survived for millennia.
Plague of Justinian
The first major plague pandemic, accepted as the first epidemic ever recorded with reliability, was the Plague of Justinian in 541. The plague takes its name from Justinian I, who was the Emperor of the Byzantines during the outbreak, and who contracted the virus himself (but survived). The capital of the empire, Constantinople, was the hardest hit city during the epidemic and reported up to 300,000 deaths in the first year alone (up to 10,000 per day). Historical sources claim that the plague originated in sub-Saharan Africa, and eventually took between 25 and 100 million lives over two centuries of reoccurrence around the Mediterranean. These theories were generally accepted until recently, when new evidence emerged that the plague was likely brought to Europe by the Hunnic peoples of the Eurasian Steppes. Studies from 2019 also argue that there is a lack of evidence to support the estimated death tolls outside of Constantinople and suggest that this pandemic was on a much smaller scale than had previously been thought.
The second (and most famous) plague pandemic arrived in Europe as the Black Death in 1347. The disease had been brought west by a Mongol invasion of Crimea, before fleeing Italian merchants then transported it back to Italy, via many ports along the Mediterranean. From here the plague then spread around Europe and North Africa and back through Asia. The epidemic peaked for five years in Europe and killed up to 200 million people across the globe. Although the Black Death began to relent in the 1350s and it never became endemic to Europe, the plague was regularly re-introduced to various parts of the continent over the next five centuries, with major cities such as London and Moscow being repeatedly hit, and some areas even experienced a higher death toll in later epidemics than during the Black Death. By the mid-sixteenth century, plague epidemics disappeared from most of Europe, but remained in North Africa, the Middle-East and Eurasian Steppes until the late 1800s. One theory for the higher death toll during this pandemic was the ability of human fleas to retain the virus, leading to an increase in pneumonic plague cases, which was more communicable and lethal.
The Third Pandemic
The Third Plague Pandemic, and arguably the most important in terms of what we know about the virus today, began in China in the 1850s. It was contained in the Yunnan region for a few decades, before socio-economic factors, such as the Panthay Rebellion, caused a wave of refugees to spread the disease to other areas of China. From Hong Kong, British merchants then transported the disease to other parts of the empire, with India suffering the heaviest losses due to the plague. During this pandemic, scientists in Hong Kong successfully identified the Yersinia pestis bacteria, and confirmed a relationship between rats, fleas and the spread of the disease. This pandemic continued until the second half of the twentieth century, with outbreaks in all inhabited continents.
Plague in the modern world
A plague vaccine was developed in 1897, which helped to contain the disease in India and during subsequent outbreaks in the 1900s. The number of plague cases fell sharply throughout the twentieth century, due to improvements in medicine, sanitation, and the extermination of natural reservoirs. A very small number of plague cases continue to be reported today, with 95 percent occurring in sub-Saharan Africa and Madagascar. Between 2010 and 2015, there were 3,248 plague cases reported globally, resulting in 584 deaths. There was even one case reported in the US in 2018. Today, plague outbreaks are fairly easy for authorities to manage, and the disease can be treated easily with antibiotics. However, unlike diseases such as smallpox or measles, which have or are being eradicated through vaccination, the plague's naturally occurring presence in colonies of wild rodents make it very difficult to eliminate in nature, and it is unlikely that the disease will be completely eliminated in the coming decades.
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