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The social history of viruses describes the influence of viruses and viral infections on human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago, when humans developed more densely populated agricultural communities. This allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences. Smallpox and measles viruses are among the oldest that infect humans. Having evolved from viruses that infected other animals, they first appeared in humans in Europe and North Africa thousands of years ago. The viruses were later carried to the New World by Europeans during the time of the Spanish Conquests, but the indigenous people had no natural resistance to the viruses and millions of them died during epidemics. Influenza pandemics have been recorded since 1580, and they have occurred with increasing frequency in subsequent centuries. The pandemic of 1918–19, in which 40–50 million died in less than a year, was one of the most devastating in history. Louis Pasteur and Edward Jenner were the first to develop vaccines to protect against viral infections. The nature of viruses remained unknown until the invention of the electron microscope in the 1930s, when the science of virology gained momentum. In the 20th century many diseases both old and new were found to be caused by viruses. There were epidemics of poliomyelitis that were only controlled following the development of a vaccine in the 1950s. HIV is one of the most pathogenic new viruses to have emerged in centuries. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They drive evolution by transferring genes across species, play important roles in ecosystems and are essential to life. ==In prehistory== Over the past 50,000–100,000 years, as modern humans increased in numbers and dispersed throughout the world, new infectious diseases emerged, including those caused by viruses. Earlier, humans lived in small, isolated communities, and most epidemic diseases did not exist.〔Clark, p. 56〕〔Barrett and Armelagos, p. 28〕 Smallpox, which is the most lethal and devastating viral infection in history, first emerged among agricultural communities in India about 11,000 years ago.〔Villarreal, p. 344〕 The virus, which only infected humans, probably descended from the poxviruses of rodents. Humans probably came into contact with these rodents, and some people became infected by the viruses they carried. When viruses cross this so-called "species barrier", their effects can be severe, and humans may have had little natural resistance. Contemporary humans lived in small communities, and those who succumbed to infection either died or developed immunity. This acquired immunity is only passed down to offspring temporarily, by antibodies in breast milk and other antibodies that cross the placenta from the mother's blood to the unborn child's. Therefore, sporadic outbreaks probably occurred in each generation. In about 9000 BC, when many people began to settle on the fertile flood plains of the River Nile, the population became dense enough for the virus to maintain a constant presence because of the high concentration of susceptible people.〔Tucker, p. 6〕 Other epidemics of viral diseases that depend on large concentrations of people, such as mumps, rubella and polio, also first occurred at this time.〔Clark, p. 20〕 The Neolithic age, which began in the Middle East in about 9500 BC, was a time when humans became farmers.〔Barker, p. 1〕 This agricultural revolution embraced the development of monoculture and presented an opportunity for the rapid spread of several species of plant viruses. The divergence and spread of sobemoviruses – southern bean mosaic virus – date from this time. The spread of the potyviruses of potatoes, and other fruits and vegetables, began about 6,600 years ago.〔 About 10,000 years ago the humans who inhabited the lands around the Mediterranean basin began to domesticate wild animals. Pigs, cattle, goats, sheep, horses, camels, cats and dogs were all kept and bred in captivity. These animals would have brought their viruses with them.〔McNeill, p. 71〕 The transmission of viruses from animals to humans can occur, but such zoonotic infections are rare and subsequent human-to-human transmission of animal viruses is even rarer, although there are notable exceptions such as influenza. Most viruses are species-specific and would have posed no threat to humans.〔Baker, pp. 40–50〕 The rare epidemics of viral diseases originating in animals would have been short-lived because the viruses were not fully adapted to humans〔McNeill, p. 73〕 and the human populations were too small to maintain the chains of infection.〔Clark, p. 57–58〕 Other, more ancient, viruses have been less of a threat. Herpes viruses first infected the ancestors of modern humans over 80 million years ago.〔Crawford (2000), p. 225〕 Humans have developed a tolerance to these viruses, and most are infected with at least one species. Records of these milder virus infections are rare, but it is likely that early hominids suffered from colds, influenza and diarrhoea caused by viruses just as humans do today. More recently evolved viruses cause epidemics and pandemics – and it is these that history records.〔 The influenza virus is one that seems to have crossed the species barrier from pigs to ducks and water fowl and hence to humans. It is possible that a fatal plague in the Middle East at the time of the late 18th Dynasty was associated with this transmission at Amarna. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Social history of viruses」の詳細全文を読む スポンサード リンク
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