Disaster scenarios as we often see them in sci-fi movies, really happened and humanity has lived in the past of great pandemics which have wiped out a large part of the world’s population. The plague triggered by a bacterium, a bacillus Hersinia pestis and the worst of all past pandemics. The Black Death of the 14th century has reduced from 30 to 50%, in just a few years, the people of Europe, from Asia and North Africa. According to the regions, one in four, one in two and sometimes even more were swept away by this terrible disease. Imagine the consequences of a contagious disease that would reduce the number of people in your country, from your continent and the world, half in just two or three years. Imagine the demographic consequences, economic, political, social and psychological that this would engender. This has happened at least twice during the so-called Justinian plague of the sixth century and during the Black Death of the 14th century, not to mention the countless recrudescence of this disease until 18 centuries. How historical pandemics of plague, did they go off? What are their origins? Their demographic and social consequences? How have they changed the history of the world? How can a bacterium evolve and become so virulent? Discover the mechanisms that have allowed the bacterium Yersinia pestis to become so dangerous to better understand the processes of appearance and propagation other emerging diseases and other pandemics such as Ebola, Flu and other bacterial and viral infections? A study published in the prestigious scientific journal Cell, in october 2015, brings new lighting on the origins and evolution of this monstrous and abominable disease. WHAT IS PEST? The word plague derives from the Latin word pestis which means scourge. It was used in antiquity and even in the Middle Ages to designate a set of major diseases, who were considered a punishment of god and who killed a large number of people, such as cholera, typhus, smallpox and plague. Several pandemics have been reported by historians since antiquity, but only two of these have been attributed with certainty to bubonic plague. This disease was easily recognizable by these symptoms, the most obvious of which is the appearance of buboes, hence its name bubonic plague. All the witnesses of that time report the appearance of buboes around the neck, behind the ears, the armpits and groin of the victims. The plague is caused by a bacterium, more exactly a bacillus called Yersinia pestis. This bacterium and transmitted by flea bites, who feed on the blood of the black rat rattus rattus. If he’s stung, the person falls quickly ill. The symptoms of the disease, called bubonic plague appear in humans, after an incubation period of two to six days, sometimes a little more. The plague form transmitted by the plague is characterized by a high fever, a general weakening, vomiting, haemorrhage, diarrhea and swelling of the lymph nodes in the area stung by the flea, with the appearance of buboes, hence its name bubonic plague. It is fatal if it is not treated with antibiotics in 60 to 80% of cases. This form of disease, can evolve if it reaches the lungs to another form even more deadly, pulmonary plague, which she is very contagious. It is transmitted from human to human, by the airways by coughing and sneezing. It is fatal if it is not treated 100% of the time and leads to death in 30 to 50% of cases even with antibiotics. In the absence of effective treatment, a general infection with the bacillus of the plague, or sepsis sets in, which almost always ends in death after three days. Since when does the plague exist? In a scientific publication published in the journal Cell in 2015, Simon Rasmussen, a researcher from the University of Copenhagen and his colleagues analyzed the DNA of Yersenia bacteria, which causes plague on 101 skeletons of individuals, found in Europe and Asia dated between 5000 and 3000 years ago. They detected the presence of the DNA of the bacterium responsible for the plague in the teeth of 7 of these individuals. The first known traces of the plague in Europe are therefore dated to 5000 years. The humans of the Bronze Age could already be infected by this terrible disease, nevertheless with a notable difference and very rich in teaching about the evolution of this disease, that we turned over later. The first plague proved with certainty is the plague known as Justinian. Even though it’s probably not the first, it is the oldest known major pandemic caused by the bacillus Hersenia pestis. This pandemic bears the name of the emperor of Constantinople of the time. It is estimated that he has killed more than 25 million people only around the Mediterranean basin. The chroniclers of the time report that the plague started in Egypt in 541 then would have reached Constantinople in 542, where the number of victims would reach between 5,000 to 10,000 deaths per day. It would have passed from port to port following the commercial shipping lanes from the time to win all the shores of the Mediterranean. A study, published in the prestigious scientific journal The Lancet in April 2014, performed on the skeletons of dead people of that time, confirms with certainty that bacterial DNA detected in the teeth of these skeletons is that of the bacillus Yersenia pestis. But also has shown that the origin of the pandemic is not African. The strain found as that of the Great Black Death of the 14th century would be native to Asia like all subsequent pests. Following the path of commerce, which at that time were mostly maritime and fluvial. It reaches the western shores of the Mediterranean, Italy North Africa, Gaul and enters the lands we follow rivers such as the Rhone. It also reaches Syria and all of Mesopotamia, as well as Persia. Afterwards, the plague returned with a certain regularity, without reaching the severity of the first push, approximately every decade until the year 767. Then disappear completely without knowing how and why for almost 6 centuries. Historians believe that the plague would have changed the map of the world permanently. By weakening the Byzantine Empire, he would have lost a large part of his territory with the Muslim conquests a few centuries later and would have facilitated the total annihilation of the Persian Empire by the Arab armies. In the 7th century, under the reign of Caliph Omar, Muslim armies which have already occupied a large part of the Arabian Peninsula and the fertile crescent, were on their way to attack Constantinople and put an end to the domination of the Byzantine Empire in the East. 25,000 soldiers of the Muslim army who sat in Syria die of the plague, almost 10 times more than during the battle of Yarmouk. A theological debate began. Caliph Omar ordered Muslim armies to turn back and not to go further north. Which probably saved Constantinople from Muslim domination for another 7 centuries. The most famous and deadliest of the epidemics is the Black Death. Between the year 1347 and the year 1352. It has decimated 30% to 50% of the population of Europe, from North Africa and Asia. It was in the eyes of the people who lived it, the incarnation of an absolute plague, it was the apocalypse of the end of the world. The historical testimonies all agree to indicate that the point of departure of the Black Death in the West would be the city of Caffa, in Crimea in present-day Ukraine in 1347. This city at that time was a Genoese counter. The Mongols who besieged the city are affected by the plague, and recorded a large number of victims. Let’s see that their seat was doomed to failure, they adopted a new type of war, what could be called the first bacteriological war of history? They decided to catapult the corpses of their fallen soldiers above the walls of the besieged city. The plague was beginning to spread in the city. To escape, the Genoese boats take off, but some of the crew were already sick. They call at Constantinople, in Messina, in Gene, in Marseille, then to the Balearic Islands. As soon as the epidemic broke out, the boat was leaving for the next port, which only spread the terrible disease throughout the western Mediterranean. But the Mongolian troops served only as vectors for the disease. Its origin is much further in the steppes of Central Asia, in present-day Uzbekistan. There was a spread of the bacillus in 1320 in Mongolia. The epidemic quickly reached the Chinese Empire. Texts of the time report that the population of China has brutally collapsed, under the effect of the frightful disease. Between the year 1320 and 1340, it has grown from 130 million to only 90 million. The plague then wins the shores of the Black Sea, following the roads of silk. The ports served as a point of entry to the bacillus, which spreads everywhere by following the rivers first, then penetrates deeply into the Lands. One after the other, the big cities of the time are reached. Marseille in 1347, Paris in 1348, England in 1350, Scandinavia, Germany to return to his starting point in Russia and Asia in 1352. All the statistics despite their big uncertainties show that the population of Europe has almost been cut in half, at the end of the epidemic. Some tax records and parish registers of the time, give an idea of the magnitude of the epidemic which affected all age groups and social classes. An example in Bremen in Germany, 70% of the population died, 50 to 60% in Hamburg. These numbers are staggering, but it is very difficult to estimate the exact number of victims. Historians speak of an average of 30 to 40% of the population of Europe, between 25 and 40 million deaths, out of a total population of 80 million. There have been new plague returns approximately every decade, until the 18th century. According to statistics reported by Olivier Guyotjeannin, in his book entitled Population and Demography in the Middle Ages, the demographic decline following the famine of the beginning of the 14th century and the plague, is such that Europe in the year 1400 and half the population of the year 1300. There is no precise estimate for the number of victims in the Middle East and North Africa, but all the testimonies describe apocalyptic scenes. It is probably very fair to estimate that the number of victims of the plague as everywhere else around the Mediterranean is 30 to 40% of the original population. Whatever the numbers, the plague pandemic is certainly the biggest disaster that humanity has known in such a short time. The economic effects, demographic, social and psychological are hard to imagine. The scale of the disaster was such that people thought it was the end of the world. People were dying in the hundreds and thousands, at home and in the streets. Whole families were decimated. It was said that there were not enough living to bury the dead, who for lack of space were often buried on top of one another in mass graves. In both the Muslim and Christian world this plague, brutal and universal is considered a fatality, a destiny, a punishment of the god, anger guesses for the sins of men. As during the plague of Justinian, rather, the origin of the disease was attributed to miasma, that is to say, the air and polluted water and especially a punishment divin. Even though we knew that contact with the sick favored the disease, we thought it was because people were breathing the same corrupt air. The consequences of such a disaster are barely imaginable. This led to a total disorganization of society. The depopulation of villages and their desertification has resulted in an abundance of agricultural fields, which cut off the supply chains of cities and women, causing an increase in food prices, followed by many cases of famine. Some social layers holding the know-how are destroyed, causing a shortage of manpower. Survivors were asking for salary increases which were unthinkable a few years earlier. The pathogen responsible for the plague has been isolated, observed and first identified in 1894 by Alexander Yersin, a former student of Pasteur. This is the bacterium Yersinia pestis. He also discovers that it is the black rat that is the animal reservoir of the disease, but could never find the vector agent which transmitted rat disease to man. Yersin will quickly develop a vaccine against the plague, but who has revealed himself and who is always very inefficient. Paul Louis-Simon, a French biologist by noticing insect bites in the victims will highlight him in 1898, the role of the flea in the transmission of black rat disease to humans. To understand how the bacillus of the plague suddenly becomes so virulent and how such plague pandemics become possible. It is necessary to know the biology of the different actors of this disease, the black rat, the bacillus Yersinia pestis and the rat flea. As the published study showed by the researchers in the scientific journal Cell in 2015, the bacterium had been present for at least 5000 years, and already touched the human populations of the Bronze Age. But the genetic study of the strain found in these skeletons shows that the bacterium was far from having the virulence we know today. She has evolved a lot to adapt to these hosts, the rat, the flea and the man. How? During the last 5 millinals, Yersinia pestis acquired two genes, she did not have before. According to these researchers, without these genes, its transmission between humans, could only be done by air, or digestive, as is the case with another very close bacterium, Hersinia pseudotuberculosis. Around the year 1000 BC, Yersinia pestis has acquired, the ymt gene, which allowed him to adapt and survive in the digestive tract of the flea, normally very hostile. Without this gene, the bacteria could not have been transmitted by the flea. She also acquired the pla gene, which allowed him to colonize the different human tissues and go through the blood and lymph. The acquisition of the gene pla has greatly increased the virulence of the bacterium Yersinia pestis. A dozen bacteria that have it are enough to kill a mouse, while it takes a million in the absence of it. According to these studies, mutations have to happen, around the year 1000 BC. From this date, Yersinia has become capable to cause major pandemics of bubonic plague, like the one humanity has known. The bacillus of the plague will even change the behavior of the black rat chip, Xenopsylla cheopis to ensure its survival. How? Xenopsylla has the characteristic of having a thorny formation in the beginning of his digestive tract. When the chip steals a sceptecemic host, dying, Yersenia pestis migrates and multiplies by millions at this location. The bacteria eventually create a blockage of the digestive tract at this location. As soon as the flea of the black rat sucks the blood, she vomits and passes millions of bacteria Yersinia pestis in the blood of the host. A bite of black rat flea can inject 1 million bacteria. Furthermore, as the flea can no longer feed normally, hungry, she stings frantically, any available host thus increasing the chances of spreading Yersinia pestis. It’s the strategy (very smart) adopted by the bacterium Hersinia pestis to increase its chances of survival. Many mammal species have chips that are specific to them. The man is no exception. There is also a human flea, it’s Pulex irritans. With modern hygiene measures, it is in net regression, but it is still present among the poor and homeless. Several works carried out during a plague epidemic in Morocco at the beginning of the last century, would tend to show a transmission role of not only the human flea, but also lice as vectors of the disease. Is the plague still a danger? Vaccines used so far have all been shown to be very ineffective and have serious and even fatal side effects and have no efficacy against pulmonary plague. If they are administered on time, we can still cure plague with antibiotics. According to the statistics of the American CDC, the mortality rate is 66 to 93% without treatment with antibiotics and 16% even with antibiotic treatment. The only real protection against the plague is the prevention of the disease. Even though she does not make as many casualties, the plague is not eradicated as is the case with smallpox. It is even considered a re-emerging disease. Since the beginning of the 20th century, the number of countries registering plague cases is constantly increasing and becoming endemic in some of these countries. Plague epidemics occur sporadically in South America and Asia, but the continent that records the most cases is Africa. In 2013, The World Health Organization has documented more than 783 plague cases worldwide, of which 126 deaths. In the years 2014 and 2015, the most affected country, Madagascar has recorded more than 335 plague cases. Plague is most prevalent in Third World countries, but sporadic cases of plague are recurrent even in developed countries. The CDC counted 15 cases of bubonic plague in the USA in 2015. Should we fear a new pandemic of plague? The genome of the bacterium gives it an incredible adaptation character. The majority of known strains of Yersinia pestis, fortunately for us is sensitive to very common antibiotics. But in 1995, in patients with bubonic plague, a multidrug resistant strain has been discovered. Plague pandemics, the Spanish flu, SARS and other current and future diseases, gives us a lesson in humility. The man despite all these progress, is just a species, like another. Nature will always have the last word. You find a list of books and articles used for the development of this video in the comments. Thank you for your attention and for the next video.