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مستخدم:Basma Abdelatty/ملعب

من ويكيبيديا، الموسوعة الحرة

هذه نسخة قديمة من هذه الصفحة، وقام بتعديلها Basma Abdelatty (نقاش | مساهمات) في 01:10، 21 مارس 2015. العنوان الحالي (URL) هو وصلة دائمة لهذه النسخة، وقد تختلف اختلافًا كبيرًا عن النسخة الحالية.

20th and 21st centuries

At the turn of the 20th century, evidence for the existence of viruses was obtained from experiments with filters that had pores too small for bacteria to pass through; the term "filterable virus" was coined to describe them.[1] Until the 1930s most scientists believed that viruses were small bacteria, but following the invention of the electron microscope in 1931 they were shown to be completely different, to a degree that not all scientists were convinced they were anything other than accumulations of toxic proteins.[2] The situation changed radically when it was discovered that viruses contain genetic material in the form of DNA or RNA.[3] Once they were recognised as distinct biological entities they were soon shown to be the cause of numerous infections of plants, animals and even bacteria.[4]

Of the many diseases of humans that were found to be caused by viruses in the 20th century one, smallpox, has been eradicated. The diseases caused by viruses such as HIV and influenza virus have proved to be more difficult to control.[5] Other diseases, such as those caused by arboviruses, are presenting new challenges.[6]

As humans have changed their behaviour during history, so have viruses. In ancient times the human population was too small for pandemics to occur and, in the case of some viruses, too small for them to survive. In the 20th and 21st century increasing population densities, revolutionary changes in agriculture and farming methods, and high speed travel have contributed to the spread of new viruses and the re-appearance of old ones.[7][8] Like smallpox, some viral diseases might be conquered, but new ones, such as severe acute respiratory syndrome (SARS), will continue to emerge.[9] Although vaccines are still the most powerful weapon against viruses, in recent decades antiviral drugs have been developed to specifically target viruses as they replicate in their hosts.[10] The 2009 influenza pandemic showed how rapidly new strains of viruses continue to spread around the world, despite efforts to contain them.[11]

Advances in virus discovery and control continue to be made. Human metapneumovirus, which is a cause of respiratory infections including pneumonia, was discovered in 2001.[12] A vaccine for the papillomaviruses that cause cervical cancer was developed between 2002 and 2006.[13] In 2005, human T lymphotropic viruses 3 and 4 were discovered.[14] In 2008 the WHO Global Polio Eradication Initiative was re-launched with a plan to eradicate poliomyelitis by 2015.[15] In 2010, the largest virus, Megavirus chilensis was discovered to infect amoebae.[16] These giant viruses have renewed interest in the role viruses play in evolution and their position in the tree of life.[17]

Smallpox eradication

Rahima Banu, the last person known to have contracted smallpox, in 1975.[18]

Smallpox virus was a major cause of death in the 20th century, killing about 300 million people.[19] It has probably killed more humans than any other virus.[20] In 1966 an agreement was reached by the World Health Assembly (the decision-making body of the World Health Organisation) to start an "intensified smallpox eradication programme" and attempt to eradicate the disease within ten years.[21] At the time, smallpox was still endemic in 31 countries[22] including Brazil, the whole of the Indian sub-continent, Indonesia and sub-Saharan Africa.[21] This ambitious goal was considered achievable for several reasons: the vaccine afforded exceptional protection; there was only one type of the virus; there were no animals that naturally carried it; the incubation period of the infection was known and rarely varied from 12 days; and infections always gave rise to symptoms, so it was clear who had the disease.[23][24]

Following mass vaccinations, disease detection and containment were central to the eradication campaign. As soon as cases were detected, the victims were isolated as were their close contacts, who were vaccinated.[25] Successes came quickly; by 1970 smallpox was no longer endemic in western Africa, nor, by 1971, in Brazil.[26] By 1973, smallpox remained endemic only in the Indian sub-continent, Botswana and Ethiopia.[22] Finally, after 13 years of coordinated disease surveillance and vaccination campaigns throughout the world, the World Health Organisation declared smallpox eradicated in 1979.[27] Although the main weapon used was vaccinia virus, which was used as the vaccine, no one seems to know exactly where vaccinia virus came from; it is not the strain of cowpox that Edward Jenner used, and it is not a weakened form of smallpox.[28]

The eradication campaign led to the death of Janet Parker (c. 1938–1978) and the subsequent suicide of the smallpox expert Henry Bedson (1930–1978). Parker was an employee of the University of Birmingham who worked in the same building as Bedson's smallpox laboratory. She was infected by a strain of smallpox virus that Bedson's team had been investigating. Ashamed of the accident and having blamed himself for it, Bedson committed suicide.[29]

Before the September 11 attacks on the United States in 2001, the World Health Organisation proposed the destruction of all the known remaining stocks of smallpox virus that were kept in laboratories in the US and Russia.[30] Fears of bioterrorism using smallpox virus and the possible need for the virus in the development of drugs to treat the infection have put an end to this plan.[31] Had the destruction gone ahead, smallpox virus might have been the first to be made extinct by human intervention.[32]

Measles

Before the introduction of vaccination in the US in the 1960s there were more than 500,000 cases each year resulting in about 400 deaths. In developed countries children were mainly infected between the ages of three and five years old, but in developing countries half the children were infected before the age of two.[33] In the US and the UK, there were regular annual or biannual epidemics of the disease, which depended on the number of children born each year.[34] The current epidemic strain evolved in the first part of the 20th century – probably between 1908 and 1943.[35]

Reported cases of measles in England and Wales from 1940 to 2007 showing a drop from 400,000 annual cases to less than 1000.

In London between 1950 and 1968 there were epidemics every two years, but in Liverpool, which had a higher birth rate, there was an annual cycle of epidemics. During the Great Depression in the US before the Second World War the birth rate was low, and epidemics of measles were sporadic. After the war the birth rate increased, and epidemics occurred regularly every two years. In developing countries with very high birth rates, epidemics occurred every year.[34] Measles is still a major problem in densely populated, less-developed countries with high birth rates and lacking effective vaccination campaigns.[36]

By the mid-1970s, following a mass vaccination programme that was known as "make measles a memory", the incidence of measles in the US had fallen by 90 per cent.[37] Similar vaccination campaigns in other countries have reduced the levels of infection by 99 per cent over the past 50 years.[38] Susceptible individuals remain a source of infection and include those who have migrated from countries with ineffective vaccination schedules, or who refuse the vaccine or choose not to have their children vaccinated.[39] Humans are the only natural host of measles virus.[37] Immunity to the disease following an infection is lifelong; that afforded by vaccination is long term but eventually wanes.[40]

The use of the vaccine has been controversial. In 1998, Andrew Wakefield and his colleagues published a fraudulent research paper and he claimed to link the MMR vaccine with autism. The study was widely reported and fed concern about the safety of vaccinations.[41] Wakefield's research was identified as fraudulent and in 2010, he was struck off the UK medical register and can no longer practise medicine in the UK.[42] In the wake of the controversy, the MMR vaccination rate in the UK fell from 92 per cent in 1995, to less than 80 per cent in 2003.[43] Cases of measles rose from 56 in 1998 to 1370 in 2008, and similar increases occurred throughout Europe.[42] In April 2013, an epidemic of measles in Wales in the UK broke out, which mainly affected teenagers who had not been vaccinated.[43] Despite this controversy, measles has been eliminated from Finland, Sweden and Cuba.[44] Japan abolished mandatory vaccination in 1992, and in 1995–1997 more than 200,000 cases were reported in the country.[45] Measles remains a public health problem in Japan, where it is now endemic; a National Measles Elimination Plan was established in December 2007, with a view to eliminating the disease from the country.[46] The possibility of global elimination of measles has been debated in medical literature since the introduction of the vaccine in the 1960s. Should the current campaign to eradicate poliomyelitis be successful, it is likely that the debate will be renewed.[47]

Poliomyelitis

Hospital staff examining a patient in a tank respirator "iron lung", during the Rhode Island polio epidemic of 1960

During the summers of the mid-20th century, parents in the US and Europe dreaded the annual appearance of poliomyelitis (or polio), which was commonly known as "infantile paralysis".[48] The disease was rare at the beginning of the century, and worldwide there were only a few thousand cases per year, but by the 1950s there were 60,000 cases each year in the US alone.[49]

During 1916 and 1917 there had been a major epidemic in the US; 27,000 cases and 6,000 deaths were recorded, with 9,000 cases in New York City.[50] At the time nobody knew how the virus was spreading.[51] Many of the city's inhabitants, including scientists, thought that impoverished slum-dwelling immigrants were to blame even though the prevalence of the disease was higher in the more prosperous districts such as Staten Island – a pattern that had also been seen in cities like Philadelphia.[52] Many other industrialised countries were affected at the same time. In particular, before the outbreaks in the US, large epidemics had occurred in Sweden.[53]

The reason for the rise of polio in industrialised countries in the 20th century has never been fully explained. The disease is caused by a virus that is passed from person to person by the faecal-oral route,[54] and naturally infects only humans.[55] It is a paradox that it became a problem during times of improved sanitation and increasing affluence.[54] Although the virus was discovered at the beginning of the 20th century, its ubiquity was unrecognised until the 1950s. It is now known that fewer than two per cent of individuals who are infected develop the disease, and most infections are mild.[56] During epidemics the virus was effectively everywhere, which explains why public health officials were unable to isolate a source.[55]

Following the development of vaccines in the mid-1950s, mass vaccination campaigns took place in many countries. In the US, after a campaign promoted by the March of Dimes, the annual number of polio cases fell dramatically; the last outbreak was in 1979.[57] In 1988 the World Health Organisation along with others launched the Global Polio Eradication Initiative, and by 1994 the Americas were declared to be free of disease, followed by the Pacific region in 2000 and Europe in 2003.[58] At the end of 2012, only 223 cases were reported by the World Health Organisation. Mainly poliovirus type 1 infections, 122 occurred in Nigeria, one in Chad, 58 in Pakistan and 37 in Afghanistan. Vaccination teams often face danger; seven vaccinators were murdered in Pakistan and nine in Nigeria at the beginning of 2013.[59] In Pakistan, the campaign was further hampered by the murder on 26 February 2013 of a police officer who was providing security.[60]

AIDS

Left to right: the African green monkey, source of SIV; the sooty mangabey, source of HIV-2; and the chimpanzee, source of HIV-1

The human immunodeficiency virus (HIV) is the virus that – when the infection is not treated – can cause AIDS (acquired immunodeficiency syndrome).[61] Most virologists believe that HIV originated in sub-Saharan Africa during the 20th century,[62] and over 70 million individuals have been infected by the virus. By 2011, an estimated 35 million had died from AIDS,[63] making it one of the most destructive epidemics in recorded history.[64] HIV-1 is one of the most significant viruses to have emerged in the last quarter of the 20th century.[65] When, in 1981, a scientific article was published that reported the deaths of five young gay men, no one knew that they had died from AIDS. The full scale of the epidemic – and that the virus had been silently emerging over several decades – was not known.[66]

HIV crossed the species barrier between chimpanzees and humans in Africa in the early decades of the 20th century.[67] During the years that followed there were enormous social changes and turmoil in Africa. Population shifts were unprecedented as vast numbers of people moved from rural farms to the expanding cities, and the virus was spread from remote regions to densely populated urban conurbations.[68] The incubation period for AIDS is around 10 years, so a global epidemic starting in the early 1980s is credible.[69] At this time there was much scapegoating and stigmatisation.[70] The "out of Africa" theory for the origin of the HIV pandemic was not well received by Africans, who felt that the "blame" was misplaced. This led the World Health Assembly to pass a 1987 resolution, which stated that HIV is "a naturally occurring [virus] of undetermined geographic origin".[71]

The HIV pandemic has challenged communities and brought about social changes throughout the world. Opinions on sexuality are more openly discussed. Advice on sexual practices and drug use – which were once taboo – is sponsored by many governments and their healthcare providers. Debates on the ethics of provision and cost of anti-retroviral drugs, particularly in poorer countries, have highlighted inequalities in healthcare and stimulated far-reaching legislative changes.[72]

Influenza

Members of the American Red Cross removing a victim of the Spanish influenza from a house in 1918

When influenza virus undergoes a genetic shift many humans have no immunity to the new strain, and if the population of susceptible individuals is high enough to maintain the chain of infection, pandemics occur. The genetic changes usually happen when different strains of the virus co-infect animals, particularly birds and swine. Although many viruses of vertebrates are restricted to one species, influenza virus is an exception.[73] The last pandemic of the 19th century occurred in 1899 and resulted in the deaths of 250,000 people in Europe. The virus, which originated in Russia or Asia, was the first to be rapidly spread by people on trains and steamships.[74]

A new strain of the virus emerged in 1918, and the subsequent pandemic of Spanish flu was one of the worst natural disasters in history.[74] The death toll was enormous; throughout the world around 50 million people died from the infection.[75] There were 550,000 reported deaths caused by the disease in the US, ten times the country's losses during the First World War,[76] and 228,000 deaths in the UK.[77] In India there were more than 20 million deaths, and in Western Samoa 22 per cent of the population died.[78] Although cases of influenza occurred every winter, there were only two other pandemics in the 20th century.[79]

In 1957 another new strain of the virus emerged and caused a pandemic of Asian flu; although the virus was not as virulent as the 1918 strain, over one million died worldwide. The next pandemic occurred when Hong Kong flu emerged in 1968, a new strain of the virus that replaced the 1957 strain.[80] Affecting mainly the elderly, the 1968 pandemic was the least severe, but 33,800 were killed in the US.[81] New strains of influenza virus often originate in East Asia; in rural China the concentration of ducks, pigs, and humans in close proximity is the highest in the world.[82]

The most recent pandemic occurred in 2009, but none of the last three has caused anything near the devastation seen in 1918. Exactly why the strain of influenza that emerged in 1918 was so devastating is a question that still remains unanswered.[74]

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